How do I gain pecs/abs?

I'm a sort of scrawny 13 year old, and I need to get pecs/abs. I already have a vague six pack when I tighten my abs, but it'd be nice to not have to tighten them. As for the pecs, I'm almost completely flat there. Also, I don't have time to go to a gym, nor the money for a membership. What would be a good diet and exercises?