I agree with jyl- get a '90's steel mountain bike with no suspension from Craigslist for $100. Get a helmet, lights, lock, pump, spare tube, patch kit for another $200.

The crummy Walmart bikes (Next, Pacific, Mongoose) are no good. Brands to look for are Specialized, Trek, GT. Look for 21 speeds or more; then it's likely you'll get a freehub (stronger) in the back rather than a freewheel (weaker). Check for no looseness or funny grinding feeling when you spin the wheels, cranks and turn the handlebars. Check for easy shifting, smooth riding, good braking. Find a friend that likes bikes and learn some mechanics or join the local co-op for mechanic-ing pointers and cheap parts.