what am i supposed to do about mood-shifts?

I can be confident at times, but I can quickly lose all confidence just by being around a lot of people, specifically girls. Here is the scenario; I am at the beach, going by myself cause I want to body board and none of my friends are into it, whatever, i go to the beach and it's all right, but i don't really know if i am physically appealing to the girls i'm into (the beach babes hehe). I don't know what i am supposed to do, my confidence about my appearance is sometimes way high, but when i am around people i want to appeal to, i begin to self-analyze and in the end mess up whatever plans i had. This sucks. What I want to do is get a girlfriend, but I have no clue on how to communicate with them. I'm 17, which is even more stupid because I should be going for the girls. This is a lot of crap i'm writing down, but I really want to know from a Girl's point of view on what i am supposed to do in situations where it's just me and i want to talk to you, and you may or may not be with your friends, ANYthing from a girl will be nice. THANKS