Exercises for abdominal muscles?

I want to do exercises to achieve well defined abdominal muscles (six pack).

I know that you have to eat really nutritious as much as you train. But I already have almost no body fast, because I have a really fast metabolism.

Also, when I tense my abs, I can already see the creases where they are supposed to be, even though I never worked out my abs. And I’m 13. But don’t say I’m too young, because I know several kids in my school who already have a six pack.

I tried the bicycle crunch, but it’s too hard to do perfectly. I always mess up with the pedaling motion with your feet.

So is there any other good ab (and oblique) exercise, that doesn’t require equipment?