I'm a new DM and i'm preparing using helpful YT vids. Do you have any good tips to run a session?

Quote from YamiYugi_1027 >>

I'm a new DM and i'm preparing using helpful YT vids. Do you have any good tips to run a session?

 To be fair you're asking an extremely broad question here, if you narrow it down, people may be more likely to respond.

Since this is pretty broad, I'll just tell you the kind of things that I learned about DMing that were pretty important early on.

1. Make sure you understand the rules well. When I started out, I had no idea what anything was, so we had a "bleed" mechanic and it was horribly OP. Killed the game. 

2. Give good descriptions! Don't drag them on for too long, but make sure that they are enough for players to envision themselves. I played with another new DM once, and they weren't the best at this. It meant that we couldn't really improvise any solutions, etc. 

3. Try to make combat settings interesting. Don't always have the ground be a flat open space, maybe it's inside a building, and there's a table between you and the enemy. Then the barbarian wants to flip the table over at them. Let that happen, it's fun! Or maybe it's on a ship, and their are oil lamps above. Spell or ranged attack misses? Oil lamp gets hit, and everything below it in a certain radius takes fire damage when it falls down! This also kind of relates to my last point, where if you don't give players a whole lot to play with it's not as fun.

4. Don't railroad! It really sucks as a player when you think up a cool solution that should work, and the DM just says no because that's not the way things should go. Give the players options! Let them be creative! Never expect that they will do what you think they will. This kind of relates back to that other DM I talked about. They put my party in the middle of a desert to start, and we saw something in the distance. So we moved closer to it. Couldn't figure out what to do, since the DM's descriptions didn't really give us much to go off of when we got there, so we wanted to leave and search for a city or town. We weren't allowed, and the campaign kind of died there. 

5. I don't know how you pick your players, but if a player isn't really cooperating in some way, make sure that you communicate with them and the rest of the party to solve the issue. If you find a group of random people online, and one person starts doing weird things and making the party uncomfortable, don't be afraid to at least discuss it with the party or kick them out. Dnd should be fun, and you can't have fun when you don't feel safe. 

Nylethen above covers pretty much everything, but I'd like to add that the first ever sessions a DM organises are almost NEVER "good". Try your best to run a campaign, and learn from mistakes you've made from the previous. So don't feel bad if your previous campaign was not very good and such, mine was too! 
New DMs tend to face issues involving the players (but this entirely depends on what party you're dming for), there are many videos online to deal with stuff like that and make sure to take it slowly and steadily. Try a session once a week approach, so you can catch up and have time to prepare for the next. 
A lot of DMs may suggest a 'Session Zero', this is basically just a session that is made before the campaign actually begins to establish what you expect, rules, what the characters of your party involve, etc etc. This makes things a whole lot simpler rather than just jumping blindly right into your story. 
Here's an optional video on Session Zeroes if you want: https://www.youtube.com/watch?v=2MA-z5Ai-bQ&ab_channel=DungeonDudes

Have fun on your session! 

1st) Know the basics of the rules. 

-If you haven't played the game before, then play four or five sessions before you DM.

2nd) Use modules to help teach you how to world build, set up and populate dungeons and quests. 

-5E does not have much I can recommend for world building types of modules.  The best I can give you is Phandalin. 

-Its extremely light on descriptions of the rooms, which helps to set the tone.  It has bare minimum of a quest line and it overuses its regional map for the amount of encounters.  You can get it to go all the way up to levels 14 I believe.  The drawback is for the most part, the creators did the 5 room dungeon, so they are small.

-Phandalin is the best that 5E offers for a Western Marches Campaign, unfortunately.  It was a very effective campaign mechanism used in prior editions to great effects.  Players love it.  Look it up on Youtube for an example.

3rd) Don't tell players how to run their characters, let the world tell players how they run their characters. 

-If a guy is playing a bully and he bullies the townsfolks, the party will find the shops will likely try to avoid them and will shut down if they are present.  When they come down the lane tell them a bell is ringing and shops start to close.  People look upon the party with fear.  You hear whispers of "them" and "dark ones".   

-If a guy decides to go on a killing spree against civilians and the party joins in, send in mercenaries to kill them if they are in a kingdom.  If they are in the wilds, congrats they can join the local orchish tribe and use their subpar goods and deal with threats against them in town.

-If players kill the merchants at night and leave no trace, well they won't have a problem now will they.  The townsfolk won't know and they got some extra coin.  Treat it as a roleplaying game.  Sloppy players get sloppy results, good players get good results and smart evil players get filthy lucre.

4th) Don't let players run other players characters after they've learned the basics of the game, see 5 below. 

-This one can be hard, but if you have pushy player telling everyone how to play outside of their turn, pull him to the side and ask him to please not talk outside of his turns and he only has 6 seconds to ask for help.

5th) Help the players for the first 2-4 sessions

-For new players give them suggestions on rolls if they don't come up with it.  For example, the party sees a dead orc with a withered hand.  They loot him and leave.  Mention to them, they could use an investigation roll to see what killed him, maybe try survival to see what killed him and use history to see if they know the orcs tribes.  

-New players are really bad at using their abilities, give them possible options for the first couple of sessions.

6th) Draw out who the big bad is in your campaign, then set up his sub captains.

-Figure out who is the bad guy and set up his sub captains.  Bring up the big bads name on occasion.  Fill the party with dread of him.  Maybe have him take one of their family or call them trash and fly off with his dragon while his army of gnolls charge after them.

-You want to know who is the leader for the part of the module and how to drop hints of upcoming events.  If you can drop them in the very 1st part of the campaign and then have them show up later, players love that.

7th) That's what my character would do

-When a player starts going down that excuse, he's probably playing a jerk.  An jerk to others at the table, the DM and the world.  They get defensive when its brought up to them as well.  It is generally a bad sign.

-Monitor the group, if they keep asking him to stop and he won't, talk to him after the session and ask him to leave.  Once its gets to the "that's what my character would do", its probably going to bad.  Try to correct it if you can, but don't be afraid to boot because they will kill your session.

8th) Buy a laminator , Cork board and an edge cutter

-use hand outs, players love them.  You have a merchant that sells potions, do they always have two healing potions for sale and its hard to keep track?  Well make two healing potion chits and laminate them with the shops name.  Tell the players they have to turn them in when they use them.

-for local quests use a cork board with laminated quests.  Let the players pick the quests and run them.

I just want to agree with an earlier poster, you’ll get better answers and support if you can narrow your questions down.  Beyond that, solid preparation always defeats the ability to make stuff up as you go along.  Improv has it’s place though, no doubt.

Well for one thing, always start with the Starter Set. It's called that for a reason. It starts off with some rules that's easy to learn in a day or two. You could make a campaign using only a stater set, but I HIGHLY don't recommend it.

Once you're ready, you can purchase the PHB (Player's Handbook), DMG (Dungeon Master's Guide), and MM (Monster Manual). These are the most important sourcebooks to make campaigns, use them, and even modify published sets. 

Now I know that I said the Starter Set came with rules, but it doesn't digitally because you use the basic rules. I have both digital and physical copies of the source books I have named (plus Dungeon of the Mad Mage and I'm thinking on getting Mordenkainen's Tome of Foes). I just wanted to make some obvious thing even more clearer. 

Make and play with optional rules, because they make your game more fun and unique, but make absolutely sure to play test or you might end up with the (fantasy) worlds biggest mess.

Also reward creative ideas by players, if it steps a bit outside the rules, use your power to arbitrate to decide how their plan ends up going. And let them make ideas that are completely doomed as well. The players have more fun when they feel that a bad idea has bad consequences. 

At the end of a session, ask players what they want to do next. 
This means you can prepare accordingly but not be railroading! Just a small little thing but it helps make sure you’re not caught off guard too often.

-Don't worry when you mess up, it's going to happen.

-Have an advance understanding of D&D Mechanics and Rules

-Accurately and quickly conclude and solve issues between players or DM to player / versa

-Start every session consistently in a similar manner of theme. Common practice is a recap. But what I like to do is write out the current day, time, environmental setting to set the starting tone.

-Learn to balance between saying No and Yes. Give too much and the players complain, give to little and they'll complain

-Let the players do a bulk of the interactions. Meet them at the halfway point when necessary. I think that if the DM is doing more of the talking, either the players are lazy or the DM is over stepping somewhere.

-Don't go into detail to how little or how much you've planned things. This can effect expectations and really, the players shouldn't be prying, especially if they are simply there to consume what you are providing, without adding any assistance in creating more content.

-Ensure that one player is not overshadowing the others. Yes, you can have a PC that is leading, but if they are doing the bulk of the talking, the skill checks (Especially when they are a class that wouldn't normally do that), its not only inappropriate, it can be kind of rude.

-Do NOT let the players bring you down if you have to postpone or cancel a session. Without you, there is no game. DMs are far fewer than players, so the players must accept your authority.

-Avoid as much as you can, excessively awarding one player over another. Find ways to get the under dog players to shine and when they bite and succeed, reward them too.

Feel free to message me for any additional input. I want to help in creating more DMs, because finding a good one, is VERY hard.