Hey, welcome to MORE! Good for you to be getting further into your riding!
Before you buy anything at all, I'd encourage you to expose yourself to as many different bikes as you can. Go to your LBS--my personal rec for the Fairfax area is The Bike Lane--but also show to some of the MORE rides and see if guys won't let you throw a leg over their bikes.
I agree with Jabberwocky that you should avoid buying used unless you have an experienced rider who can help you hook up with the right ride. But I'm not sure I agree that you should avoid an "old" bike, so long as it's steel. (And, again, you've got that ringer to help you get a good one.) Aluminum has a shorter shelf life, so it's probably safer to go with steel. You can probably get a MORE member to sell you an old steel steed for a song, and doing that might buy you some time to figure out how to spend your money on a longer-term solution.
Speaking out ofthe other side of my mouth, if you're not inclined to learn how to do your own wrenching that might be a good reason to buy new.
The main thing, IMO, is to avoid spending an arm and a leg on your first nice ride. Get something that matches you as a rider, and expect to upgrade as you grow into the sport.
And have fun with the process. The hunt for your next bike is one of the thrills of the sport