Just like a battle plan, nothing ever survives the first shot.
Always be prepared to think of stuff up on the fly. A good DM will be able to think up of stuff on the fly and come up with ideas.
Trust me, even if you have thought out of everything the players will find a way to bypass your "fullproof" game.
Now, also character creation will probably take way longer than you think as it always does so I try to have everyone have their characters done before hand.
And don't think of this stuff as intimidating, just make sure that you have some little ideas in the back of your head.
Also running a module might seem like a great idea but unless you want to sit down and read the module completely through I try to shy away from them, as it bogs down the game since the DM tries to flip through pages and pages of stuff to try and find the correct thing to do. If it's your own homebrew stuff then you know everything and can push the game along at your own pace!
*edit* ninja'd by Ginafae :p, and if you do like to do pre-generated stuff and just roll with what the players do then maybe that will be a better idea. I know I don't work well with modules, so maybe I'm just biased :p