I'm sure that plenty of people will say the same, but the Rule of Fun is paramount. Unless following rules to the letter is what everyone loves, don't be afraid to manipulate the rules a smidgen. I'm also a pretty new GM, but that seems to be what my players like. Also, count on your plot getting subverted, it will happen eventually.
Find out which classes your PCs will be a make some treasure aimed towards to those classes, like Imperious said manipulate the rules sometimes and have fun. My people like cheesy humor, so I throw in a Non-subtle reference here and there "The skeleton is pinned to the wall by a single arrow through its knee"
Avoid having players you or any of your other players actively dislike. Unnecessary drama is likely to ensue.
In the game, archenemies can be fun, but you don't have to use the same one throughout the campaign. If you have a secret organization/conspiracy/cult/ whatever, you can occasionally throw enemies at the party that they will soon learn to automatically hate and/or fear and recognize by their facial tattoos/distinctive cloaks/aberrant traits/spiked armor/whatever.
Allow players to fail if they deserve to fail. A hollow victory is no victory at all. Players want to believe, when they succeed, that they did so against the odds. This is a very important illusion for you to foster, but it falls to pieces if they sense that the game is on Easy mode.
If a rules question comes up and it looks as if it may take a while to resolve, unless a PC's life is on the line, try to jury rig a provisional solution and tell the players you will come up with a more permanent ruling for future situations out-of-game. Then do that. If a PC's life IS on the line, though, then either take however long it takes to resolve the question OR make a ruling in the PC's favor for this ONE occasion and tell the players that this ruling is not necessarily a precedent, that you will research it and that future rulings may or may not go the same way.
Never kill any PC whose player isn't at the table at the time, unless you KNOW they're never coming back (in which case the PC is an NPC anyway).
Have fun. It's a game.
random rolls are ur friends. I random roll whatwver loot there is in a dungeon down to usually I dm for 4 other people and I have a d4 just for seeing which player the monster atks.
Establish an outline for ur story, the settings, and get an idea of a few important npcs. Once thats good, u can usually just go off the fly with things. Trust me, sucks so bad when u spent every free moment fleshing out an encounter or dungeon only to have ur players so how come up with the most direct route. Trust me u will realize dnd players have this inborne ability to fimd an exit or staircase quickly lol.
Usually I outline a map and decide how many treasure chests are gonna be on that level. Ill random roll some loot and even random roll some encounters and even some rooms and details. I get an overall basic plot in my head and then I let them do tbeir thing. Somethings ill already have planned out and somethings I leave empty just to make up something on the fly.
Just dont waste time planning EVERY single detail or outcome bc I can bet that 90% of the stuff wont ever get used or see the light of day.
Background music is nice and sometimes it pays to just go fullblast on the detaila of the rooms and creatures. Smother them in detail. Like theres a hallway, tell the players how well lit the hallway is, how long or short it is, what decorations there are, etc etc.
One of the most useful prep rules I've found is that if the PCs are going to be in a town or heavily populated area, make a list of random NPC names. My players always randomly interact with random NPCs, and it's nice not having to think something up on the fly and slowing the game down.