What's up man. How do you know this girl? Do you just know her from seeing her around school previously? Is she a friend of a friend? Is she your cousin's neighbor? That could play a huge part in your approach.

Either way, for one thing if you've met her before, just approach her and say, "Hi you went to/you were at (insert school/place you saw her here) right? If she says yeah say, " I thought you looked familiar. I'm (your name) by the way" She'll introduce herself then just start talking. This assuming your at a different school/place now.

If not, find out her name, then approach her and say,"You're (her name) right? She'll say yes then say, "Hi I'm (your name)." Just say, "I remember you from last year but I never got to meeting you." Strike up a conversation and before you go seperate ways, tell her, "Well hey you should give me your number and we'll hang out sometime."

I'd reccommend asking her for it the 2nd or 3rd time you talk. Unless you see her once every 2 weeks, or you want to close the deal fast. Get it the first time. This way she won't say she doesn't know you, or she barely met you. Some girls are like that. They're concious about any person having their number.

Remember girls like confident guys, it'll show if you have the nerve to approach her. Go for it man. If not you'll be kicking yourself in the a$$ everytime you see her walk by thinking it's another chance lost. Good luck.