I want to get abs...?

I'm about 17 and i'm 5''0 and I weigh about 125... I have a flat stomach but its giggly and I was wondering if anyone knows if it's possible for me to get some muscle in my stomach and get that line in middle of my belly.. because I want ab lines but my goal in about 4 months is to already have muscle in my stomach and my lower abdominal as well... I do cross country running at school the most we'll run in 6 miles and the less will be 4 miles, what can I do to make my goal in 3 months!!?