My first bike came from a pawn shop for 75 dollars. My wife picked up a nice Univega at a Goodwill store. When I bought my first bike I knew nothing at all about them.
I lucked out: my pawn shop bike fit me like it was custom made for me and turned out to be a decent quality bike. I rode that bike for six years. My wife had the advantage of my experience.

I would repeat the craigslist advice offered above. I would also check out lawn, tag, porch, garage or whatever sales that they may have in your area. Most importantly make sure that it fits you. You can change almost anything but the frame. Make sure it fits.