How to talk to this girl I like? 10 points to best!?

So sometimes my crush walks over to me and my friend at lunch (she doesn't know my friend) and she watches us. I want to talk to her and I think she wants me to too, but I don't know how. I always get nervous and freeze up and panic and I think its obvious :P. What should I say I've never done this before I'm only a high school freshman. Help? Long answers would be appreciated. Oh and she already sorta knows me, but she doesn't know my friend, which is why I think she is coming over. To talk to me, not for my friend.