I recently joined a new Pathfinder role-playing campaign run by a relatively new GM. After the first session I was impressed by how much prep she had done for the game. After the second session I was blown away. And really, preparation is the key to being a great GM.
While players may put their own spin on your ideas, even avoiding a well thought out encounter entirely, a well prepared GM is ready to go with the flow and re-tool an encounter on the fly. The thing is that confidence comes from being well prepared. If you know what monsters you want to use you’ll know what rules to brush up on. If you brush up on the rules your combat will go more smoothly. If your combat runs smoothly then the game will be more fun. Being prepared is just a win win situation for all.