Im 14 in high school and I really like this guy but he has a girlfriend. But I feel like he likes me too.. What do I do?

Nothing. He has someone already. How would you feel if you were dating a guy and some girl came along and tried to steal him away? Wouldn't be nice, would it? So, you just let him go and maybe one day, he'll become available.

You leave him alone. If he liked you, he would be with you.