Be a better Dungeon Master than The Dungeon Master. That guy was kinda crazy.
You’ve been playing RPGs for awhile, but it’s time to run a game.
Are you nervous? Don’t be.
As tough as it seems to juggle all those monsters and numbers and encounters and characters and whatever, it’s not really. At least, it shouldn’t be.
And this advice applies to any game or edition you’re playing, though I personally feel like D&D 5e makes the DM’s job a little easier and more flexible.
Your job as Dungeon Master is simple: Help the players tell a fun, exciting and engaging story, and adjudicate rules when they come up.
Beyond that, we have some more specific advice.
Start simple. Unless you want to spend hours doing so, you absolutely don’t have to dive head first into your own campaign world. It’s hard to come up with a compelling world full of quests and dungeons and characters. Go ahead and start with a published adventure. They’re fun and most of the hard work is done for you. (May I recommend the D&D Starter Set? It’s got literally everything you need to play a game right in the box, and the adventure in there is pretty fun.)
Be prepared. Read through the adventure first. Have the rules and books and handouts in front of you, just in case you need them. I like to build a little binder (read more about that here) with everything I need, and I keep my Player’s Handbook, Dungeon Master’s Guide and Monster Manual nearby even if I never end up needing them.
Be fair. Everyone at the table should be treated equally. Everyone should be able to shine. Everyone should be having fun. This is especially when a player comes up with a crazy way to defeat a monster or disarm a trap or otherwise derail your plans. If it works, and the dice roll true, let them do it. Even if it takes the story down an unplanned path, roll with it and see what happens. Sometimes those are the most fun games.
Be unfair. We’re not trying to contradict ourselves. Honest. But sometimes, you need to fudge things a little bit if a rule or a dice roll or whatever is going to derail your game. And you’re allowed! If a fight with a few orcs is going too easily, maybe some of their friends show up to turn the tide. If you meant for the players to break down a door, but they can’t seem to do it, you can mentally reduce a DC to help them succeed. Just don’t do it too much. (See previous paragraph!)
Don’t sweat the rules. The rules are there to help tell the story. If the story would be better by throwing one out you don’t like or alter a rule that’s giving you trouble, by all means do so. Even the guys who created D&D 5th edition ignore rules they don’t like.
Don’t worry about looking in the book. Last week, I watched D&D’s Stream of Annhiliation quite a bit. As I watched, I noticed that Chris Perkins has to look stuff up in his Player’s Handbook. A lot. You know what? Me too. All the damn time. Perkins is basically D&D’s chief storyteller, and I more or less consider him the master DM. If he has to look up rules and spells, you shouldn’t worry about it either.
You don’t have to be a voice actor. Watching the folks on Critical Role get into their characters is pretty fun, but you don’t need a clever voice for every NPC. Even if you do a voice, you don’t have to remember it exactly for next time. You don’t have to be an actor. I’m currently running Curse of Strahd, and I give every character the same terrible Transylvanian accent. Unless I forget and don’t give them a voice. It’s awful. Guess what? My players don’t even seem to notice.
You don’t need to have miniatures and maps and tokens and all that. Trust me. I have all those things, and half the time I never get to use them. For starters, I play a lot of my games online via virtual tabletop sites like Roll20 and Fantasy Grounds. And when we do play in person, I end up using a lot of the same stuff over and over. Just as fun: A blank grid you can draw on with paper tokens you print at home OR running things theater of the mind without maps or minis.
You’re not trying to win. Your job isn’t to beat the players. (Theirs isn’t to beat you, either.) You’re all telling a story together. Pushing the players exactly where you want them to go all the time is called railroading, and no one likes that.
It’s OK to be wrong. You may make a ruling during a game, and find out later that you interpreted the rule wrong. That’s fine.
Say “yes” a lot. If a player wants to try something, let them. Set up conditions they have to meet, and then see if they can follow through. They may succeed, but they may not. But don’t shoot them down out of hand. If they want to do a flip over an orc while slashing at his throat, have them roll an acrobatics check, see if the orc will hit them with an attack of opportunity and have them roll an attack. Maybe they take down the orc in a flash of brilliance or maybe they end up flat on their back with the orc glowering over them. Either way, it will be memorable.