What Smith6612 suggested is probably the most effective way of getting to know a girl. Everything starts with common interests. If you can find something that she's interested in and you're genuinely interested in, then bring it up to her. For example, if you saw her skateboarding (I know, weird example) and you enjoy skateboarding, just go up to her and say, "hey I saw you skateboarding the other day... what kind of board was that?" That way you'll have something more than "Hi" to say to her and it would make the situation less awkward.
Just have confidence in yourself. If you're ever involved in a conversation with her and other people, be sure to talk at least a little bit, even if it isn't directed to her. Hope I've helped a little.