Try both methods and see what works. Trial and error is okay. For a lot of people, learning how to GM means figuring out what are the aspects of game you can improvise, and what need to be planned. Which ones you can improvise is going to be unique to you.

The best advice I can give is don't bite off more than you can chew. The biggest mistake I've seen players make when they run their first game is trying to run something too big. They're used to starting at level 1 or only playing vampires in WoD, so they tell their friends to make a level 15 character or that they can play werewolves, vampires, and demons in the same game. They do this because as a player it's the sort of thing that seems fun, without realizing it's unmanagable as a GM, especially so as a brand new GM.