Hey fellow NOVA commuter -- I ride 14.5 miles into DC and another 14.3 or so back each day. It's a great area for riding - and there's quite a few of us on here.

For commuting, my take is that you can basically use anything that's comfortable to you. I see people on mountain bikes, hybrids, road bikes (even with aerobars), touring bikes, and just about everything else that's out there.

Figure out your frame size - that's the 1st thing to do. People argue about minor aspects of frame size, but you darn-sure want to get a frame that fits you well.

Do you have a source of anything used that may fit your body? I'm thinking of friends/family in the area with something....

Craigslist is sometimes a good source for bikes that are much cheaper used than you'll find new. Big Wheel bike shop (my personal favorite) in Alexandria has quite a few used bikes that are mechanically sound and reasonably priced (150-300 bucks).

I have 2 bikes here in DC: A surly long haul trucker (touring bike - roadish but with fatter tires/fenders, racks, panniers) and an old 1985 road bike with skinny tires. They are quite different, but both work just fine for reliable commuting. The road bike is faster and more nimble, but is not as great in the rain and doesn't carry gear as easily. On the other side it's so ugly and old that it's not a theft-target in DC nearly as much.

It's just personal preference.

Do a search here and you'll hit millions of other questions just like yours.

Get a helmet and some water bottles.

Get a front and rear blinkie light -- probably a bell too for alerting trail users, I love mine.

Get a high-quality U-lock and a cheap cable lock -- regardless of the bike you choose. Sheldon's locking advice:

http://www.sheldonbrown.com/lock-strategy.html