Just say hi.

Yea, it's that simple. Look at what she's doing, If she's reading a book, talk about the book. Get a little interested in the book and see if she reads books often and what kind?

Or if she's writing something, ask her about that.

Anything she seems interested in at the moment, ask her about it.

If she has a weird shirt on, or pants on, something with writing, make a comment on that(Try to keep it respectful, no matter how lewd the shirt is.)

Basicly, Just seem interested in her.

Another way of going about this is to find someone she and you both know, and ask that person about good topic starters and how to approach her in general.

If you have the same class as her, you could ask her for help in that subject, but that only works well if she's really good in the class.