Girlfriend advice for a socially awkward 16 year old?

I'll be honest: I've never had a girlfriend in my life and I'm almost 16 years old. I've turned down 2 opportunities because I was so nervous and didn't know what to do even if I had said yes. You could say I'm "socially impaired" tbh. I get really, really nervous around people, especially girls and ESPECIALLY ones I like. I don't know what the par is and I don't know how to react to anything in a relationship really. I would be the person that on a 4th date at a movie would be putting my arm up and back down 20 times trying to decide if I should make the move. Seriously. I'd be a nervous wreck the week before prom, assuming I could even manage to ask a girl out to prom.

I have had what you might call a half-girlfriend long-distance before and it started with facebook chat and eventually I started calling here every week, but I got so nervous that half the call turned into awkward silence as I tried to think of something, anything to say. Eventually I just stopped calling altogether because I didn't know what it was supposed to be and I thought she probably deserved more than 1 awkward telephone call a week. If that was the wrong course of action someone tell me but I didn't want to embarrass myself by "breaking it off" when I didn't even know if she considered us that.

Not even really sure what I'm asking here... I guess I'm asking what to do if I have the opportunity and I know the girl likes me back... if I asked her out what then? I just don't know...

A girl's perspective would be nice here.