For your first few stories you'll need to have details, but less than you might fear. Have NPC names, professions and dispositions worked out ahead of time. For combat you'll want to know what they'll fight and how that creature acts. It is a good idea to write out a stat block for the creatures until you know them well. This is because as a new GM you'll be thinking about the monsters tactics, the players tactics, and just how bad that last roll was and whether you need to refill your soda before or after combat. The fact that this creature can do a loopdeloop when it hasn't moved this round can easily get lost in the wash. Worse, you don't want to be halting combat to scan the entry for the creature the whole time.
So... have the basic plot outlined, enough NPCs to make that plot advance, and know well all the baddies the chars will have to face down. Do no more than that, and you'll do great/ But also keep in mind that this is a game, and the players are your friends, fun is the goal.. so.. the most important thing is not to stress.