Have you heard of Sean T(?)'s Insanity? They only use their own body weights. It has given people crazy results without lifting weights.
In fact, try it with things like pushups and plank. You only use your own body weight and do them at an incline- thus, chances of injuries are greatly reduced. Go slow. Either raise your legs off the floor, or raise your hands off the floor. They don't hurt your back, and generally improve your upper and forearms, shoulders, between neck and shoulders in upper back, fingers, and wrist muscles. Additionally, to a lesser extent: abdomen, thighs, lower back, and buttocks. That covers well over 3/4 of your body. Try the pushup challenge with us.
Similarly, you can do things like pull ups that can also work out your torso and arms.
Try pilates, although generally seen as women's domain-they are probably one of the best things you can do for your body. Pilates is also regaining ground amongst men.
And as others said, everything else is from proper nutrition.
I don't use weights.