Trick to GMing isn't the world building; it's dealing with other players and being able to walk a group of players down an IC path. Sounds simple, a thousand little issues come up along the way. 

But unless you're trying to GM an Advanced or High Casual game right off the bat, you'll do fine as a first time GM, more than likely. And if you don't, get more experience as a RPer first, and try it again in a month or so. 

Simple, easy.