Dungeons and Dragons is a wonderful pass time with pretty much endless possibilities as far as what players can actually do. However, the entire experience hinges on a single person: the Dungeon Master. Running a game as a DM can be nerve-racking, as the entire experience depends on how well they can paint a picture for their players. But, like all things, it gets easier over time, and there are quite a few things beginner Dungeon Masters can do to make sure that they, and their players, are having a good time.
4 Let Players Explore
First off, your campaign is going to take a detour at some point. There are a bunch of reasons for this, maybe players misinterpreted a clue or would rather just mess around for a little bit. And that's okay! D&D is about letting players have fun in the world the DM creates, and it's important to allow them that freedom. If the players get too off track, rope them back in with a random event tied to the narrative or give them a challenge they aren't quite ready for, but can easily escape. The key is to prepare a compelling narrative, but leave yourself room to improvise when it comes to the broader world. Keep a scratch pad with names you can throw out whenever they meet an NPC, but leave enough space to take notes about them so that they can come back in the future. Make sure that the main narrative is well-paced and thought out, but there's still spontaneity- it'll make for a much better adventure.
It can be easy to get caught up in the minutia of a game world, but it's important not to invest too much time into that. Instead, think about the general atmosphere of different locations in your world, then use that to build experiences. The important thing is to take notes so that you always have information to come back to later on. It'll make the world feel more real, and encourages players to explore the world.
3 Don't Be Afraid To Homebrew
As DM, what you say is final. If there's anything about a rule that you dislike or doesn't mesh well with your game, change it. Some players may protest or believe that it's unfair, but at the end of the day they do have to respect your decisions, otherwise, the game will devolve into anarchy. This is called homebrew, and it ensures that the game will always have unique twists that separate it from other campaigns. And it doesn't stop with the rules, any content you create or change is considered homebrew, and you should use it as much as you like. The only important thing to remember is consistency. Make sure that if a rule applies to one player, it applies to them all, otherwise there's sure to be accusations of favoritism, which never ends well. Of course, there are circumstances where you may want to break established rules, but it's better to stay consistent most of the time.
2 Follow The Rule of Cool... Most of The Time
The time to break established rules is when the Rule of Cool comes up. At its base, the Rule of Cool means that a DM should allow their players, or at least give them the chance, to accomplish something incredible but outside the realm of normal possibility, usually due to how well they roleplay a situation. This can require some suspension of disbelief, but it'll keep the players invested and more willing to come up with creative solutions to their problems, which keeps the game fun. Don't overuse the rule of cool, as doing so can make the game feel campy and the world feel unrealistic- unless that's what you're going for, in which case, run wild. Otherwise, a good rule of thumb is once per session, though that's incredibly flexible.
1 Give The Players What They Want
This may sound obvious, but it's something many DM's get wrong when they first start running games. If your players prefer combat, give them combat encounters. If they like to roleplay, give them opportunities to roleplay, and even avoid combat situations by roleplaying well if that's something they dislike. Each player is different, so it's important to strike a balance that pleases all players at the table. This will take a while, and that's alright. Being a good DM takes practice, but designing the right balance of encounters will start to feel more natural over time.
It'll take a while to find your groove as a DM, but once you do, you'll be able to create world-class experiences. It's okay to take a break now and then too. Give other players an opportunity to DM a one-shot, or even run their own mini-campaigns. It'll let you recharge, inspire more ideas, and give you a chance to figure out what does and doesn't work for your players. At the end of the day, just have fun, and don't let being a Dungeon Master feel like a part-time job. It takes a lot of work to prepare sessions each week, and it can be very easy to get burnt out.