How to talk to a girl?

Hi i want to increase my social skills at school. i am a guy in gr8. I am pretty sociable at school, and a good reputation. I have a lot of guy friends at school, although im kinda shy to talk to girls. Im not shy to talk to girls in my class, but the girls that are not in my class, or perhaps only in one of my glasses, i would be a little bit shy too talk to. I am kinda like a nerd at school, i get good marks, but i try to act cool and stuff. Mostly girls just ask me for homework help, nothing else, and it kinda ticks me off cause i want to talk to them, not talk about homework. Well i really like this girl at school, shes pretty and i heard she is really nice. She is not in any of my classes, nor any of her friends. She will never be in any of my classes cause she is in french immersion, and i dont do french french immersion. So im to shy to talk to her by the lockers and stuff. What do i say? i don't want to talk about things an adult would talk about, you know teenage stuff. i dont want to make it awk. What do i do? I tried talking to her like once, but we didn't talk more than 16 seconds. Like i said i can talk to girls in my class, but how do i talk to girls that are not in my class, and make sure they don't think im a nerd and stuff? People say just to be myself and man up, but i can't i always screw up and be awk. Any ideas? P.S. in Gr.9 there is a chance i can be in a class with this girl i like but not 100%