Prepping for the start of a campaign is an exciting time, full of possibilities before the realities of the PCs tear everything down.
Ok, it's not actually that bad... most of the time.
It does describe one of the things you really have to watch out for as a DM - falling in love with your campaign before it begins. Inevitably it will morph, mutate even, the longer it goes on. Players will do things that you're not going to expect, and that you're going to have to deal with. It's normal, if sometimes painful. Just keep that in mind as you get started, and remember that it isn't just your campaign. Every second of table time makes it a shared experience.
The beginning of any campaign can be found in the seed of an idea. Whether the seed becomes a thriving living thing, or withers right away will depend on the buy-in of your players. It doesn't really matter how awesome or original an idea it is, if your players don't like it, they're not going to play it. They might show up at the table and roll dice, but the will either be unenthusiastic, or they will try to take the game in a different direction from the one you had in mind.
Talk to your players. Sell them on the idea. Have an elevator pitch ready to go, and really, seriously sell it. It doesn't matter how outlandish or standard it is: Clockwork golems finding allies to defeat the evil priest kings, or murder-hobos looting a sprawling ancient tomb. See if this sounds like something your players would be interested in playing, and don't be afraid to work with them to develop something everyone will enjoy: Pyro-goblins in the sprawling dungeons driving off the ancient undead priest kings from their tombs.
Once you have buy-in from your players, you need to decide how open a game it's going to be: Sandbox, Rail Road, or something in between? Maybe for the first game it'll be something a little more defined until everyone gets a feel for the game. Maybe everyone's agreed to play the railroad. Either way, this is where you'll prep what you'll need for the first session, and write down some ideas to follow up on depending on how things go in the first session.
That sounds really organized, doesn't it? In reality, while I spend an awful lot of time thinking about it, and making notes (incomprehensible even to me the next day), I usually go in pretty unprepared unless I'm running something someone else wrote, and then DM by the seat of my pants. When I get really organized, I've usually over-prepared, much to the determent of the game.
Post 7 in the 30 Days of GMing Challenge.