So let me preface this that I am by no means an expert DM, gods gift to the craft or anything of the sort. I don't have 30+ years of DM'ing either I just recently talked to a couple friends who wanted to DM their own little one shots or campaigns but found the idea daunting and I thought of what to say to them and figured I'd share the same with the internet for whom it may concern.

1.) Know your table - There are a lot of talks about how to run a campaign the do's and dont's and very strong feelings about some of those opinions. The first is the taboo talk of rail-roading. Of course as a DM you want your players to feel free to run around talk to every villager, chicken, small shrubbery they find and have it all mean something. However I think it's important to know who you DM for. Some players (like my friends in the first campaign I ran) were happy to chomp down on the first hook i through and let me reel them through story beat after story beat seldom walking off the beaten path. Other people want to walk into a city and meet every guard, guild member, merchant and vagrant you can think of. I think regardless of what people may tell you is the "proper way" to present a D&D story figure out or talk to your players and figure out what all of you are looking for. Some people want grand adventure others want furry highschool. And that's ok!

2.) Set expectations early - It can be hard sometimes especially in the digital era of tabletops to know what to expect. With access to all ranges of players with varying levels of experience, it can be hard to know who should and shouldn't be at your table. This ones a bit different from the first point because this touches on what you want to feature in your campaigns. Some people want Matt Mercer, Brennan Mulligan, etc. to be their DM and unfortunately you probably aint that. That's ok though truly! You simply need to have those conversations. Also you may want to include things in your campaign that people may not be comfortable touching whether it be racial tensions (dwarves hate elves, humans hate orcs etc), diving into theology (questioning faith and gods), or anything else. When handled with care they can offer great story at times but some people simply don't want that in their D&D and that's ok too. Make sure you talk to your players.

3.) Resources - This one's pretty self explanatory really, from random roll tables (useful when someone goes hey whats that guys name sitting at that table 3rd from the left, loot,random encounters) DM's guild resources to handle things 5e doesnt really cover (like how there is no half elf half orc), sites that offer battle maps and the like. Make sure to curate these and find what you really like.

Well for now thats what I got. I might add more later but for now a couple things to consider with the final thing being, DON'T BE AFRAID TO SUCK AT IT. Being bad at something is the first step to being great.

Also if anyone reading this wants to add their own thoughts do your thing.

On top of those here is what I wish I knew before I started

1: Be ready to improv. The players have no idea what you want to do story wise, so they WILL go off course. Be ready to legit pull npc's out of nowhere and find simple lures back to the plot

2: Take notes, if more than a one shot. Nobody even thought to tell me to take notes so it didn't cross my mind until i needed a detail from 3 sessions ago and i couldn't remember. TAKE NOTES

3: get to know the monsters you want to use before you use them. The CR rating system is terrible at times, take the shadow for example. CR 1/2 monster, it has resistance to every damage, and can drain strength to the point of out right killing players if their strength score hits zero. 

Descriptions: six senses: "feel", taste, touch, smell, see, hear. Always include smell, see, hear, and feel when describing.

Your Players are the greatest source for ideas -- even when they don't realize it. Listen to them, know them. Know what scares them, what they love.

every movie and TV show can become and adventure if you want it to.

Celebrate and remember the awesome moments -- have a bard singing about it in a tavern one night, and wath the PC get played up. 

Give everyone a chance to be a hero.

The fun and the moment matter more than the roll.

write little notes out ahead of time. use index cards or Private messages. 

You don't have to be able to act out everything. You just have to make it feel like you do.

bend, but don't break.

The thing I wish I knew was how unpredictable players can be and that no amount of preparation is enough. They ignore locked doors (and they absolutely fail to discover the hidden ones, never put anything important behind hidden doors), they spend a long time discussing the religion with the random peasants…

Especially with the adventure books you need to be ready to improvise a lot.

1) Have a Session Zero. This really helps set up expectations about what the campaign's going to be like and what styles of play will be involved. A Session Zero can go a long way to establish expectations, get everyone on the same page, and learn what themes your players are not comfortable in game.

2) Let Your Players Guide the Story. As a Dungeon Master, you make the obstacles and design challenges, but you let your players decide what they want to do and how they want to deal with threats. Let their actions guide and shape the plot and force the villains to move out of their normal comfort zone and start responding to the character's actions.

3) The Game Is Just for, Fun. People play D&D because they want to laugh around the table with friends and have fun. Your goal is to allow your players to enjoy themselves and their experience at the table. If they're having fun playing the game, then you must be doing something right. If you think you've designed something really cool but your players don't like it, then move on to something they find more enjoyable. Likewise, if you've made something  that didn't seem great to you, but your players are loving it, then you might want to let them continue to explore that plot line.

Though you want your players to have fun, you should be able to have fun too. If you aren't enjoying the game, you have every right to leave or stop running it. Your player's fun doesn't have to come at the cost of your enjoyment of the game, and it makes sense to at least run plot lines, adventures, and stories that you actually like playing/running.