Help with getting some more muscle? More distinctive abs?

I know this question is commonly asked but I just can't seem to find an answer that works for me. I am 13 years old, 5'6" and I only weigh 87 lbs.! I cant stick to some super diet and I don't own any weights. I already work out but nothing works! I eat all of the food I need to! I am looking for more toned abs then i already have, distinctive pecs, forearm muscle, and upper arm muscle. Any good advice?