How to tone and get definition in your lower stomach muscles?

Hi I'm 14 and female. Iwant to get a more toned and defined abs. I do lots of sports e.g. athletics, football, karate, but I find it hard to get definition in my lower stomach. Just under my ribs I have really strong prominent muscle but then my lower stomach although is flat isn't toned. Any stretches, advice or just anything would be great!