Trial and error. The best way to overcome this whether it be because of fear or just lack of social skills, you have to just go out there and face your fear. Over time and with repetition of trying to be more social, eventually you will develop the social skills and be comfortable around others.
Define anti-social. Do you actively hate or dislike people and want to take action of some sort against them? That will get you in trouble; get counseling. Do you just not enjoy being with people in groups? There's nothing wrong with that. Be your own best friend.
Being antisocial in a social situation is like being non-aggressive in a fight. Your always going to lose if you wait on your opponents moves don't be afraid to strike first but stay in control. If you want to make friends strike up a conversation. If your want to get noticed strike one with who you want to get noticed by. If you want to ask someone out just throw a punch while guarding your heart with the other hand(metaphorically speaking)
You are going to have to force yourself to take the first step into a crowd. Realize the first 10-20 minutes are going to be uncomfortable but will dramatically improve as time goes on. Its the transitional phase that is most difficult, just realizing what that is will be helpful in overcoming your fears and changing a pattern.