I love this girl but, I don't know how to approach her

First of all, slow down, you've never talked to her and yet you want to marry her? Try smiling at her nicely, not too many teeth, you're not trying to bite her! and if she smiles back, say hello, and introduce yourself, then just take it from there, "faint heart never won fair lady", good luck :)

Go up to her all calm and say something smooth like "Hey there sweet thang... Can I buy you a fish sandwich?" . Works every time- it is failsafe ;)

Hmmmmm I am curious. How do you know you want to marry her if you don't really know her? Maybe you are just kidding... I suggest you start with just talking to her about anything. LOL a girl can drop something and hope the guy picks it up...maybe you could drop something near her, say oops, excuse me and as you pick it up, keep talking. Good luck :)