Why can't I get girls?

I'm 19 now and I still haven't had a girlfriend. I've been told by people that I'm good looking (not amazing but above average) but girls just don't seem interested in me. Girls always seem to look away from me as soon as I look at them and I just don't understand why. They seem to never be making eye contact with me. I mean is there something wrong with my facial expression or something? I just don't understand. I know guys who look worse than me have gotten girls. What should I be doing to get girls?