What are some good exercises without weights or machines?

The only tool I have for helping me with my exercises is gravity. I'm 15, around 130-140lbs, and about 5'9". I'm not looking for huge arms and a showy six pack, I just want strong arms and and strong abs, even if that means that they don't look it. Currently, I'm only doing push-ups, leg lifts, and crunches. There are numerous websites and forums that have numerous approaches and exercises to do, but I can never tell which is going to work. What exercises should I do? How many of each? How often? What should my diet be?
 
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