How do i talk to this girl?

Ok, I'm a junior and I really suck with talking to girls (I freeze up and get really tense when I start talking to a hot girl). I've only actually had one girlfriend mostly because girls never really take the time to get to know me. Well anyways there's this one girl (shes a freshman) that I've wanted to talk to for a while but I just can't bring myself to do it. I don't know when to talk to her and what exactly to say because I only see her once during the day and she's always with her friends which is during P.E. (she has team sports and I have weight training so I only really see her while we take attendance unless my class ends up in the gym like we usually do once a week). When we take attendance she's at the end of her line and I'm at the begging of mine (we have to stand on the wall) so I'm like 10 feet away from her. When we go in the gym she usually sits in the corner with her friends and I usually play basketball or something like that. Also after we come out of the locker rooms and when she walks by me she always whispers something to her friend and giggles, I've also seen her staring at me from time to time. When would be a good time to go over and talk to her and I really got no clue what the hell I'd say. Can someone give me some pointers on what I should do and don't leave some BS answer about ';just go do it'; or something like that because that doesn't help if you haven't noticed. Thanks everyone.How do i talk to this girl?
i used to have the same problem just tell yourself shes one of your guy friends and pretend she is in your head then when you get to know her slowly introduce it to your self and get comfortable with it