You can get into fantastic shape using your body weight as resistance. The idea is familiar to lots of us starting with situps and pushups, but there are so many variations that you can do to get a really complete full body workout.
Try
Body by Fish for some great routines that will really kick your butt. If you haven't worked out in a while, take it easy at first. It is easy to think that because it is your own body weight you won't be working that hard. If you push yourself too hard with one of these workouts, you won't be able to walk the next day. I know from experience!