I'm not social guy too, but I found out this:
Try to be original, stand out... never had a girl until I was 16, why? because I was "one of many", when I was 16 I started to be goth, wear those black clothes and generally didn't give a fuck what others think. At that time, girls started to talk with me etc.
Also don't be shy. If you are sitting next to a girl and you will kiss her, there is high chance that she won't protest! Thats how all the guyz that have million billion chicks do. They just go and kiss the girl =)
Don't try to play cool, girls can see that. Try to be more cute, do things that will make them "Aw, thats so cute :3".
Don't tell them you like them right away, you can say it, but when there were signs that she might like you too (go out with her, test how she reacts when you hold her shoulder, try to hug her when you are going home..)
Oh, and, girls aren't "sexy" they are "beautiful" =P never tell a girl she is "sexy" unless its the right situation and she wants to be called like that (like if she is wearing really short skirt and shirt)
Good luck =)