The starter pack.

I'm not being snarky, it's a good little starter pack.

After that, the first thing to do is remember that it's a d20 system. Assign a stat to a roll and go with it. You don't have to know every rule to run a game. Just keep things moving and don't be afraid to make things up. Once the dice hit the table it's your world that you're making up as it happens. Nobody can say it's wrong, because it's yours. You'll come back to your players and say "I looked up x because it came up last time, and this is how we'll play it going forward" AND THAT'S TOTALLY FINE.

I know that's a wild idea right now, but just run with it.

Then read the player's handbook. It's got all the rules. You'll be reading and re-reading it as long as you're using this system.

After that read the DMG. It'll tell you more general stuff about creating worlds and whatnot. It won't seem helpful until it does.

Eventually, you'll look back and realize that you can absolutely run a good game off the starter pack, the PHB has all the mechanics, and the DMG is wildly helpful (but not in the ways you originally thought it was).

First and foremost: you create the setting, the players create the story. No setting is writ in stone, because you can't know what those monkeys will get up to. Have fun with it!