This is a recently-answered question on Yahoo! Answers by a young man who wanted to build muscle with body weight exercises. Here's my winning reply:

I think you asked a bunch of questions there! I'll try to give a brief answer. From your question, I gather that you want to do body weight exercises, like pushups, pullups, and ab work, with a focus on building lean, quality muscle and losing belly fat.

You have somewhat divergent goals, in that you want to get bigger muscles but you also want to get leaner. At 14, you probably have a very fast metabolism (meaning you can eat a lot and burn most of it off without getting chubby). So you should try to eat more to put on muscle.

Here's what I suggest. Start with one set each of the following body weight exercises for as many reps as possible:

1. Squat. If you can do more than 20 of these, do single-leg squats instead. The idea here is that squats build the entire body, including the arms, simply because they stimulate the most growth hormone production.
2. Pull up. This will build your back (giving you that athletic V-shape) and biceps. There really is no better exercise for building tremendous biceps.
3. Push up. The ultimate chest builder. This will add size to your arms, shoulders, and chest. It will also help with the abs.
4. Crunch. No longer a popular exercise, it's still one of the most effective ab blasters of all time.
5. Leg raises. You can do these hanging from a bar or lying down. If lying down, you really should brace your lower back by placing your hands in the small of your back. These really hit the lower abs.

Keep up the other activity (swimming and playing ball) and you'll stay lean even if you eat more than the average teenager.

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