So you’ve decided that none of the published adventures are what you want to run, and you want to build your own world. Congratulations! You’re a homebrewing Dungeon Master / Game Master!
There’s several ways to build a new game, but a good way to categorise them is either top down or bottom up. The easiest method for a new DM is definitely bottom up, also called starting small.
What do you need to do to build from here? You need a location, maybe a handful of NPCs, and some immediate threat to the location. Use your Session Zero to get a feel for what things your players are looking for, how they want to build their characters, and maybe some plot hooks you can grab from character backgrounds to flesh out the area.
One player wants to play a noble, fine. The starting area is part of their family’s lands.
One player wants to play a priest, fine. There’s a church/temple/shrine in the starting location.
One player wants to play an eccentric vigilante superhero… Try and make it work.
All you need for the location is a general feel.
A large village or small town can be ideal, not too remote but not too close to a hub of civilisation.
Add in a handful of important NPCs – the sheriff, the mayor, the innkeeper, another quest-giving type – and maybe a couple just for colour or as red herrings. A travelling merchant or a friendly monster work well.
A local issue like bandits or goblins can bring a group together and let people get a feel for the rules/setting/character they’re playing pretty easily.
Drop the bad guys out in the nearby wilderness somewhere, and then have fun with it.
The goblins have moved into the area and the town don’t trust them – but they’re nice and just looking for a place to nest for the winter.
The bandits are working for someone more powerful who has eyes on conquest or needs to sow discontent for another reason – that travelling merchant could up their prices if the travel was a bit more hazardous, or maybe it’s fighters from the local militia unhappy with the sheriff or the amount he pays them.
The only big things you need to think about in this homebrew are any bigger themes that came up during Session Zero. Dinosaurs, airships, explorations of moral relativism, horror, nihilism, recent collapses of social hierarchies, or any other weird thing your players want to use, you should at least start thinking about here. Introducing at least one theme or macguffin early on will help.
If there’s some sort of war in heaven, you can flesh out the deities, but otherwise just knowing there’s coverage for the different clerics should be enough.
That’s all for now. Remember, Google is your friend when it comes to searching this stuff out.
Jame’s Haeck’s excellent article on DnDBeyond is a lot better at explaining the benefits of starting small than I’ve been above, though focuses a lot more on the doing than the theory I’ve thrown out above.
I’d also recommend jumping on this video from Matt Colville over on YouTube, which basically covers the next steps with a proto-adventure. I’ve run the adventure he describes several times, in Forgotten Realms and my own Ostromarka setting, and it really works well as an introduction to both the game and a starting adventure getting a group together.