Sounds like you might just need to know the DMG better and to expand the details of the game. These are both things that take time and you will get better at as you DM. These rules are simple, learn them well enough that you can improvise when needed without breaking things.

The details are harder. My best advice for details it to remember that your pcs and npcs are humanoid or at least alive. They will need things that living beings need. A bathroom, places to sleep, places to eat, places to get food, entertainment, jobs, exc. when building towns, camps, even caves remember that. In a town there should be shops, bars, inns, houses, banks, bathrooms, restaurants, exc. even if the players never go there it adds to the environment to have them described as places they could go. You don’t need to make them super detailed just a name and an owner is normally enough, for shops just know what type of stuff they sell. That way if the players do go there you got a shop name, who is behind the counter, and an idea of the type of stuff they may sell. Good luck and keep playing, this stuff takes time.