While it's true that almost anything will work for your application, my advice if you have any intention about getting into biking semi-seriously is to go to a bike shop. If you're new to biking, the help will be well worth it. My opinion is that the price difference between a Wal-Mart bike and an entry-level bike shop bike is not worth the difference in quality and the chance of ending up with a bike that isn't right for what you intend to use it for.
Not everything bike shops sell is 5-figure carbon and unobtainium. An entry level bike should be around $300.
That said, not everyone at a bike shop is all that helpful. They should ask you a lot of questions about you and your intended use before they recommend anything. If they don't, or can't explain why you need X instead of Y, find another bike shop.
While you can find deals on used bikes, you have to either know what you're looking at or have a friend who does to keep from paying too much for someone else's junk. I know a bit about bicycles (raced BMX as a kid and have ridden off and on for the last 15 years) and don't consider myself expert enough to buy a used bike without help.
Full Disclosure: I just dropped my 15-yo hybrid off at the shop for a new chain, front shifter rebuild, and general tune up. It's been sitting for about 5 years, and I'm going to start riding again.