These are resources that I've found helpful in preparing my games.
D&D's Official Site. This is obviously the best place you can go to find the rules. But because that's not particularly helpful, I've linked some specific pages below.
r/dndnext. I have a love/hate relationship with this community because I feel like they can sometimes act as if there's only one way to play. However, I've gotten a lot of resources from there that I use in games, so. Take the good, and leave the rest.
Matt Colville. I watched his videos obsessively before I started DMing. If I'm honest, none of this would exist if he hadn't made videos assuring me that I could do it. So watch his Running the Game series. Generators. I use a lot of random generators. Here are 3 of my favorites:
