Maybe a couple more questions for you:

-What type of riding do you plan to do? All dirt/gravel? Some paved roads? Mild/Wild?

-How tall are you? (it will help in our search for a bike frame, fitment is very important, if you don't fit well, you will now want to ride!)

Look at Hardtails (no rear suspension, only a front fork for suspension), I wouldn't suggest single speed for someone beginning, I can see it being frustrating when the "hike-a-bike" sections of your ride come sooner than expected on a ride. But they're very fun to ride, for their simplicity.

Aside from a brand bike, look closely at the components of the bike. As others have said you can get a very decent bike for cheap. but bad components can ruin a ride fast, poor shifting, lack of brakes, etc.

I would suggest that even a $250 bike would require a trip to the local bike shop for a $100 Tune-up (which is a GREAT investment! people find it worthless, don't, they know what their looking at and can make adjustments that make a world of difference) . If you're committed to some weight loss and more importantly many great memories on your bike, I think a $500 budget really opens up your bike options.

Here are a couple bikes I would feel good riding away with for the price:

19" Kona Blast Mountain bike, DISC Brakes. ***BUY-SELL-TRADE*** - $325 (Snohomish - Bellevue)

http://seattle.craigslist.org/sno/bid/4842464088.html  

17" Accesss 29er - $325 (Issaquah)

http://seattle.craigslist.org/est/bik/4825979636.html

Kona Scrap Mountain Bike - $225 (I would def do a tune-up and confirm the frame size of this one)

http://seattle.craigslist.org/see/bik/4839235914.html

Kona Roast hardtail bike - $350 (Mount Vernon)

http://skagit.craigslist.org/bik/4818033493.html