Most Helpful Girl
You can either bluntly go up and talk to her, say hi, introduce yourself, etc. or you can find a convenient excuse to talk to her and strike up a conversation.
I've heard of that book, is it good? That's a really cool necklace/bracelet/earring. It's way too hot today, isn't it? Do you know where I can find the library? If you're trying to decide on a good dressing, balsamic vinaigrette is pretty good.
Could be almost anything. Just be social. At the end say it was really nice to talk to her, maybe compliment her appearance/looks or do something to somehow indicate you're not just being friendly, and ask for her number.0