How does a 13 year old girl gain a six pack?

Okay well I am 13 going on 14 in November. I am about 5 foot 1 and weigh 90 pounds. I realize I do not need to lose weight. I want to gain muscle in my stomach. I want a six pack. I do participate in sports such as Cross Country, Basketball & Track. I am in athletics and at the moment we are doing P90X. I have a little stomach fat covering my muscles. How can I get rid of that? What should I eat? I no longer drink any sodas. Its strictly water with Powerades. What are some excerizises I can do? And how often? Thanks so much.