May I suggest that you look into buying a used bike? You'll get a lot more for your money than you will from a big-box store bike, and it will be worth upgrading. The trouble with those kinds of bikes is that they're often not put together properly. They may say "Schwinn" or have "Shimano" components, but the truth is that Schwinn makes high- and low-end stuff, and Shimano makes crazy-expensive Tour de France-worthy component and really cheap ones. The other big problem, which KatzPajamas alluded to, is that you can't really test-ride any of them.

If you've got questions--you find something on Craigslist, for example--please ask us.

I'd suggest getting a hybrid (something like a Cannondale Quick or a Specialized Ariel come to mind) or a hardtail mountain bike. Either will be good for your purposes. They're all multi-speed bikes--I'm not a huge fan of the idea of single speeds, but I live in a hilly town. How tall are you, and what's your inseam?