Like some of the other guys above, I'd suggest you drop by a bike shop to start with. Tell them where and how much you plan to ride, and they can help you get an idea of what kind of bikes would suit you ...and just as importantly, what size frames you should be looking at. You may be able to take a test ride on something and get a feel for what your preferences might be. Hopefully they'll have some things in your price range, but if not, you'll at least be better informed.

On the other hand, you mentioned cheap. If you want to go cheap, used bikes can be the best bang for the buck. Rigid (no suspension) mountain bikes in particular tend to be undervalued. You can often find good quality rigid mountain bikes from reputable brands for $100-150 or even less. The catch is that you have to be smart about what you buy. Just like when buying a used car, you need to avoid overpaying for a clunker. And that means being an informed buyer, knowing how to identify quality and knowing what you're looking for. To maximize value, you'll also need a little mechanical aptitude. Cheap bikes aren't so cheap if you need to take them to the bike shop for lots of work.

If you were to add your location to your profile so that it shows up on your posts, we've got a handful of guys around here who seem to be happy to skim your local Craigslist and point out good (and bad) bikes.