Let's get to the chase: I want to be a DM, but I don't have a strong enough idea of how it works, and I don't have any pro tips to get me started. I have a basic storyline planed, but I don't know how to fill it in with random encounters or other such things.

Any tips would be appreciated.

Thank you!

Probably not the best tip but, Im also pretty new to being a DM. I've run a one shots in the pass but I'm going to be running a full campaign soon. 

What I've pretty much done is buy the Yawning Portal book and flesh out a story arc from that. I find it's​ a really good way to see how different adventures and encounters are done and just skinny enough on story that I can customize and expand on it pretty easily. After seeing how each adventure is done in the book I've been able to get a pretty good grasp on where I can add side adventures and encounters. 

Not sure if that input helps everyone but it's worth a thought. 😄

There's books, blogs, videos, podcasts, and magazines that are all dedicated to improving the skills of a DM.  Sorting through them is tough.

Its taken years of experience for me to get where I am now, and I'm still not as good of a DM as I want to be.  You learn by playing, whether as a player or a DM, and you adapt to what you see and learn from those around you.  Just watch as the DM tells the story and ask yourself if you can do it better.  Prepare for everything.  When DMing there really is no such thing as over-preparing. You'll always feel under-prepared.

Take a rule from improv - Always agree. In improv if someone says to you, "Oh man, I heard you just got a divorce!"  You're never supposed to respond, "No I didn't... "  It ends the improv right there.  You're always supposed to agree and build upon the situation: "It turns out that my wife was a spy for the CIA. There's no way I could have stayed in that marriage!"  You build upon the suggestion that you got a divorce and you expanded the story by building upon the suggestion.  Improv, I feel, is very important for DMs.

You might have a story that you want to share with your players, but it will never go down the way you want it to.  Something will happen that you're unprepared for and you'll need to improvise.  Creating side-quests are fine, but tailor them to your players.  Use the background (one of the greatest things about 5th edition) and implement those backgrounds into your side-quests.  Not only will that make the player feel obligated to complete the side-quest, but it'll make him feel like he's achieved something.  Use your rule of improv for side-quests as well - The soldier in the group has a side-quest to find a missing captain.  The captain's wife is begging and pleading.  The player tracks down the captain, but it is a burial site (side-quest completed).  However, there is a note left upon the burial site by another soldier that suggests there was foul play.  The player tracks down the new captain.  However, the new captain turns out to be a spy for the enemy.  However, the player's investigation leads him to believe the new captain is under the influence of a spell.  The player breaks the spell and saves the day.  Keep adding to the side-quest as the players continue their adventure.  That soldier just took a side-quest that could have led him hundreds of miles from the starting location right to the front door of the big bad baddie that you intended them to fight in your main story.  Side-quests are fun like that.  You can tie them into your story nearly seamlessly.

Anyway... /rant

DMing is challenging, rewarding, and frustrating.  I love it.  

Specifically in regard to random encounters, I would suggest as a general rule plan several encounters that lean toward the easy side of the spectrum since these will usually occur prior to the players having finished a long rest, or as an interruption from the planned adventure. You can however make a random encounter list as part of the adventure. If you know the players are going to have a chance to long rest before reached the main serious of combat encounters, then you can make the random encounters harder.

Basically if the players are in the forest they run across (Roll a d8)

1-3, Nothing...maybe a squirrel or two
4, Some deer
5-6 Several wolves of appropriate CR (lean towards an easier encounter)
7 Some Bugbears who have a helpless lad tied to a tree. (Can be the start of a side quest or future event) After using have players come across Bugbear patrols.
8 A medium encounter that could be taxing on the PCs.

Normally I would lean toward not using a hard encounter, unless you want to intentionally tax the players. Random encounters are simply encounters that don't serve the main story. They don't have to be combat encounters. You could find a lost lad instead and need to help him back home.

As for more general tips, Always be prepared for the PCs doing the unexpected or not following a story point. Maybe someone offers the PCs 1,000 gold to retrieve an item, and even though you planned that the item was stolen from the individual and he wants it back, the PCs may determine that he wants to steal the item. Especially if rolls go bad. So I would say always plan 2-3 sessions ahead and always plan alternatives incase the PCs don't bite with your current plans. A couple game sessions later and you could have the PCs stumble on a caravan of thugs who have the stolen item and if they remember/recognize it, they can go retrieve the gold.

A lot of plot points may never be followed, and you kinda have to live with that as  DM. But always file everything away as it may creep up in another game, or down the road in the current campaign. Maybe the PCs didn't go save a village, and later when they find out they need something from the village, they discover it destroyed.

Try to weave future plot points into the current story. Can be as simple as meeting Delvor the Dwarf at an inn. Several months later, Devlor mentions his father has vanished.

Know the basic rules and how to handle combat. Become familiar with the PCs classes, and especially their spells and features. Nothing is worse than not noticing a PC has a spell or feature that can take your three hour plans and turn them into a five minute situation. This can happen a lot at later levels. A PC suddenly casts Polymorph on the main villain, turns him into a rat and they put the rat in a cage, fly 200 ft. above the ground and drop the rat. Charm Person and the like can also make for some interesting situations with PCs suddenly forced to reveal information without the players having to travel to the underground cavern. This can be fun from a PCs perspective, but bad if you had only prepared the underground cavern for the game session and now the PCs no longer have motivation to go there.

Tell a story. Give descriptions of the world. Describe Larry, the random bar guy who has no importance to the story and will probably never be seen again, in addition to the important NPCs. If you go, "to the right sits a man, dark black hair, appears to be missing his right hand, but wearing fine silk. A closer look and you notice his two pointed ears and a long dark sword hanging to his side" then you follow that with, "there are some other guys that are the typical sort you find in a bar." Next all the PCs walk up to the elf wondering what part of the story he has. Instead describe the elf, describe Larry, describe the barmaid. You don't have to describe everyone, but don't just describe important people.

Have a list of NPCs premade that can be dropped into the world, as well as a list of names, just in case you have to come up with something fast. Best part is that if the PCs never encounter an NPC, you can drop them into any town when the PCs do need such an NPC.

If you start at level one, try to get the PCs past level one as quickly as possible, perhaps by the end of the first game session. Level one characters are rather squishy.

 Encoutners don't have to be combat oriented. They can be social challenges. Reward XP for roleplaying, and for social encounters. If the PCs get information from a drunk guy in a clever way, reward them some xp for doing it. If it was easy, maybe 25-50 xp per person. If it was rather hard maybe even as much as 100 xp per person (assuming level 1, adjust the xp amounts accordingly as the pcs rise in level)

Take notes and always jot down a NPCs name that you happen to randomly make up during the game. You never know when PCs will want to try to find the NPC again.

Someone posted a link to Mathew Collville's DM tips which I think would be helpful to you. I've only watched the first couple, but they seem helpful and interesting, at least thought provoking. Mathew Mercer's game tips are available as well. Plus you can watch him DM on Critical Role.

I would agree that you can never be over prepared in DMing, and no matter what you think of your DMing skills, you are better than you think you are. Everyone is better than they give them self credit for in this regard.

For trying to start the main quest line, I would recommend a tavern, a guild, a royal assignment or a divine duty. This may seem cliche, but it works, I've been DMing since 3.5e and in the most recent campaign, the party started in a Tavern.

For random events, plan them ahead of time, it will save you a lot of pain and suffering. For example, if the party chooses quest A, then they will encounter a band of orcs on their way, if the party chooses quest B, then they will run into a pseudodragon who is in need of help, so on and so forth. If the party chooses to do something completely different, just take a random encounter you planed before for a different quest and throw it at them. If they missed an important letter that they needed to see to advance the plot, give it to some bandits or some other cannon fodder for the party to beat and find the letter.

There are websites you can find that have loot, encounter, dungeon generators and nearly any other type of generator you could think of, this is my personal favorite website: http://donjon.bin.sh/

Don't be afraid to change any of your plans or to make something up, you have ultimate power in your universe.

My suggestion would be to start with a released adventure (official books or Adventure League). Both are quite full of guidance for a new DM. It is the experience at the table (or virtual table) that tells you where are your strong and weak points. After a couple of adventures you can improve yourself with the channels suggested above and/or studing the DMG.

Going to agree with folks suggesting to start with a pre-made adventure. I wish I had, but I'm a sucker for making my own stuff. Generally speaking, I've never cared for "random encounters". If the players are fighting, then there needs to be a reason for it, and it should progress either the story of the game or should be imparting information to the player in some way. Even if the information is just "hey, a ton of stuff here is trapped, so you'd be wise to be extra-careful about that!", then you are giving them something to go on.

If you are married to random encounters, then you'll want to design some mini-scenarios based on locations in your story. I'm going to be doing this for a jungle-based campaign I'm dreaming up. These random encounters should have a few things:
#1 - A theme that ties back to the larger story; No matter what, these encounters need to inform the players about whatever is going on. If there's an evil force at work that's warping reality, maybe these creatures show some of that - like the fauna in this location have extra heads, or suckered tentacles where claws should be. If they're tracking down some goblins, maybe these creatures have already encountered the goblins and show signs of that.
#2 - A gimmick that makes the encounter unique: Generally, put the players in a situation where they start out as the underdogs. If they're starting with disadvantage, while the enemies start with advantage, then it's fun for both the players and for you to figure out how they can shake off those effects. Or perhaps the enemy is getting the drop on them with surprise somehow. Or perhaps the enemy has set up traps. Or something; just make it that this isn't only going to be about hacking and slashing - some complication exists.
#3 - A hint about what's to happen: The encounter needs a lead-in, whether that be having the players setting up camp, or the smell of burnt flesh in the air. Something triggers that things are about to go down. When you want there to be surprises, lead-in with something that distracts the players and/or makes them think that something different is about to happen, and then catch them off-guard.

There are lot of great tips and good advice here already. I will second the concept of "yes anding" what players say, within reason. Someone made a joke about asking the Sun Elf for a piece of Big Red because of the description I gave about the room they were in and the colors they Sun Elves wore. I thought it was funny, so I just rolled with it - the NPC, who's a high ranking government official, pulls out a beautiful silver case, opens it up, and pulls out a piece of Big Red to give to the player. Now Big Red exists in the world and it's a part of Sun Elf culture. You don't have to roll with everything, some things are just too silly or just don't work in the world you've written and you need to give a kind, but firm "no", but in general it's easy and more fun to "yes and".

I will also add that The Angry GM has a lot of great stuff - particularly the How to be a GM series. Even if you don't appreciate his sense of humor, there is a lot of gems in those articles. I took his articles on narration and scene setting and condensed them down to bullets that I turned into a mini-DM screen I use. 

Here's a few tips about group management:

• Have a Session 0. Make sure everyone is on the same page regarding the type of the world you've built, what is going to fly and what isn't (both mechanically and behavior at the table), what are the house rules, etc. It's critical that everyone knows what they are getting into and agreeing to participate in before they start. You will avoid a lot of headaches this way. Also, ask them for ideas and thoughts around what you're proposing. I'm of the mind that it's your story, but it's everyone's game. That doesn't mean let everyone walk all over you, just that you should be open to everyone's ideas.
  
  
• Communicate regularly. This is crucial. Check in with your players both as a group and individually. Ask them what parts they are enjoying and if there are any parts they aren't really feeling. You can't accommodate everyone, but you may get some good feedback that you can incorporate into the campaign or your DMing style. If you try a new mechanic or a new style of running something, ask them what they thought about it. Sometimes you'll find they like it and sometimes you'll find that an idea you thought was really cool is a buzzkill for all of the players. On the flip side, if someone is doing something that is distracting or is ruining other people's fun then you need to pull them aside and tell them that. It's the only way these things get resolved.
  
  
• Be open to feedback. When you ask what people think, you have to be open to hear whatever they say, provided it's said constructively. It's hard not to get defensive, especially when it's something you've created, so you have to work at this. Constructive criticism will make you a better DM, so I would encourage you to seek it out. If all you're getting is "It's good!" or "You're doing great!", try to dig deeper and get specifics.

As for random encounters, there is a great Monster by Environment table in the DMG and soon to be here on DDB. it's really helpful for finding a few monsters to have in your back pocket that you can throw at the party every once in a while. Also, sites like Kobold Fight Club can be very helpful for figuring out whether the party has a good shot at beating an encounter that may involve some monsters. We will hopefully have something like that here at some point too.

Finally, just run a game. It's the only way to learn. You may fail spectacularly, but if it's people that you know then they are likely going to understand that you are new at this. Remember, good players are going to want you to succeed because that means they are going to have more fun.

When you use the list of names write down the name and the location used in your notes.   Do this for all unexpected NPCs for you never know when an NPC might become a regular through no particular planning.

Also if you are in a particular area define the primary type of Encounter Beast, Orc, Goblin, Kobold, ect and have that as part of your stock NPCs on hand.   Any intelligent Race will have a territory that does not really include other Races that will fight for that territory.  As in if you are in an area of Kobolds they are not going to tolerate Goblins forces in their area except as slaves.  Same goes for Orcs.   Beasts on the other hand will have overlapping territories to a point like bears and wolves ignore borders of each other and of intelligent races.  

I agree with playing a book adventure first especially if you have new players and you're introducing them to playing.  And there's no reason why you can't have those characters go into your homebrew afterwards. Gives you a built in starting point even... "word spreads of  the groups exploits and you start receiving calls for help from local villages"  You might have to ramp up somethings if you planned your adventure for level 1 characters bur it'll give you a chance to tweak things to their play style. 

1) Have fun, don't take yourself too seriously, and roll with the mistakes. Whether you've never DMed before or you've been doing it since the Red Box, unexpected stuff and mistakes will happen. Lean into it. Some of the best moments come from embracing the unforeseeable. 

2) I'm going to echo what everyone else is saying about a pre-made adventure. It is so helpful to just see how it is laid out, and look at the pacing of it. My first time DMing I had a lovely slash and bash monster crawl, but I lacked the between bits that really make up the story. Use a premade for the first few sessions, and then implement your own writing. It's really nice to have something to reference when you do start writing. 

3) Be confident. In this world, you are the god that controls everything. Don't be a dick, but don't second guess yourself. I'd brush up on how combat works, how all of your player's special features work (somehow they always think that they can do more than is possible), and other basic rules before you start. But in the end, once you get going, you don't want to be constantly checking back at the rule book. Make a quick ruling, and get on with the game. (Also, players can smell fear.)

When running an encounter, be it a combat, role play, a trap/puzzle, what have you, try to include a description of the room or general surrounding. It doesn't have to be super detailed, just one or two unique elements to add to the area. 

Also, remember that most creatures aren't going to just fight to the death. Goblins are going to break ranks and run off into the woods if they feel like they can't win.

Dont roll for random magic items after a combat, especially a random encounter. If a creature had a magic item, why wouldn't it use that item against the characters? A healing potion is one thing. The creature just didn't get the opportunity to use it. However, a +1 long sword or a potion of haste is a game changer that creature should have, and would have, used in combat.

Id add that if a creature used a +10 sword of awesomeness in battle then the PCs should be able to retrieve the item after the battle. 

A big bit of advice is to try to talk to your players as often as you can about what they like and don't like. Don't bog them down, but ask them if they like a session, ask if there are things they want to see more or less of, and don't be afraid to let them win!

A lot of advice in his is pretty good, but I'm going to second what Lorathorn has said; talk with your players.

If you focus your efforts on making your game one that your players and you can all have fun with, then you really don't need to worry about anything else.

And while "don't be afraid to let them win" is good advice, I'll take it further and say don't be afraid to put their characters in situations where it is very likely or even nearly certain that they will win - so long as it isn't every single time they try anything, players like the feeling that there character is so good at something that it is a foregone conclusion that they will succeed at it almost always. That's especially true if it is something the player invested character resources towards, like if a rogue player chose expertise in a particular skill, it's okay if they pretty much never fail when using that skill because the character is supposed to be awesome at it.

I'll ramble on to full essay length if I don't stop myself, so this will be all for now.

My advice is, don't be afraid to make mistakes.  As long as you and your players are having fun, mistakes are just part of the game. 

I make tons of mistakes as a player, why would I not make them as a DM too?

By far the best D&D YouTube channel I've watched: https://www.youtube.com/playlist?list=PLlSmiQ728Xr2Pf3pGa3s6-zhhqpiw7hj_. Watch all the videos. They are awesome. I use most of his tips now, and the adventures are going quite well.

Running a published adventure can give a good idea of how to structure things until you get familiar with your own style. The Starter Set is great for this. 

As far as fleshing out your own story with encounters goes, this is an incredibly useful tool. Personally I prefer a narrative-heavy game style and so random encounters are my anathema. Something that might help is to think of encounters as scenes. Have a beginning (how they get into the encounter) a middle (what happens during the encounter be it combat, social interaction, a skill challenge or what have you) and an end (what happens when the encounter is resolved? Do the players successfully bypass a trap? Navigate treacherous terrain? enjoy the spoils of a victory in battle? Gain a new ally or contact?). It's also helpful to have the end of one encounter include a hook into the next encounter. Perhaps the goblins the players just slew have a map that leads to their camp or the guard they just befriended needs some help dealing with a gang of ruffians.