I'm a 20 year old guy and I have no idea what to do with my life?

This is really wearing me down. I just turned 20 in January and I am incredibly depressed. I feel so old and I feel as if I'm not really accomplishing anything. I am going to University, but I have no idea what I want to study or what I want to do with my life. To be quite honest, I don't want to grow up at all. My social life is nonexistent, because I am a painfully shy homebody, and basically all I do is study and sit at home on the computer. I don't go to parties, travel or have fun, because I have nobody to do any of those things with, and I feel like a 50 year old man trapped in a 20 year old's body. I don't want to waste my youth away and I am dreading getting older. All I do is sit around being depressed, playing video games and hating my shallow, empty existence. Not to mention that I've never had a girlfriend. Sometimes I just want to turn back time and be 12 again. I'm just not used to have responsibilities and options and I feel incredibly overwhelmed and lost.