How can a 15 year old boy develop visible abs? (six pack)?

I'm a 15 year old boy. I'm skinny, and i would really like to get visible abs. I hate being skinny with other muscular boys around me in school. I currently have a gym membership, but i haven't been for a few weeks because of money. I do a free swim after school once per week and do press ups (about 30) and sit ups (about 20-30) every night before bed. I'm also trying to cut down on fatty things such as chocolate and eating healthier foods such as fruits. How can improve in a way that will not cost too much money. Are there any good free iPhone apps out there?