Start Small...
Raduin -
I have been DMing for 20+ years and I agree with the comments made so far. To these, I would add:
(1) Read Dungeoncraft: Particularly the first 10 installments. Excellent advice for the beginning DM. You can find it
Here
(2) Start Small: Pick a small, out-of-the way village that is geographically seperated from the closest larger city or town. Use winter floods, a deadly swamp or dangerous forest to isolate the town and keep the PCs from wandering too far afield until you have a chance to work on other areas. Populate the village with the bare essentials - tavern, church, general store - and 10 or so interesting NPCs that can be mentors, political figures or even rivals of the PCs.
(3) Let the Players Help You: Give the players 2-3 paragraphs briefly describing the area and its history before they create their PCs. Ask them to include 2-3 plot hooks or ideas in their backgrounds (feel free to disallow any that are too crazy). These hooks can be great for helping your campaign to breath and grow.
(4) It's OK to Say "No": Feel free to restrict classes, races, spells or alignments. Players will often want to play something exotic or bring in spells, equipment or magic items from sources you don't have or don't plan to use. It's fine to limit the campaign to just the Core Rules initially, then bring other elements in when you feel comfortable. Likewise, it is OK to disallow prestige classes or only use those you feel comfortable with. In fact, too many choices can quickly lead to an out-of-control game, especially for a beginning DM.
(5) Allow the Players 2-3 Sessions to "Finalize" their PCs: If most of your players are new, they might not have a good grasp of game mechanics when you start playing. Allow them to swap out feats, skills and/or spells (for wizards, bards and sorcerers) - subject to your approval - after the first session or two. Make it clear the the PCs are "finalized" at the end of the 3rd session.
(6) Run a "Practice Session": Before actually starting play, have a session where you run "arena" type combats where you pit the party against a variety of enemies and against each other. This will allow them to get comfortable with thier PCs skills and abilities without it being "Do or Die"!
(7) Have a Secret or Two for Everything: Put a secret or two in place for each of your major campaign elements...some kind of little twist in the plot!
Have fun!
~ Old One