Shy girl help with friends?

[SuperiorNiglett]

So I met this girl in gym class, we had a few conversations about nothing (mainly because I'm very shy). I'm to say the least, very fond of her. She doesn't know this because it's not worth mentioning. Anyway, I think she switched out of my gym class because I haven't seen her, but I do see her in the hallway while I'm walking to gym (she's going the opposite way).

I want to get to know her better as a friend, but I don't have any classes with her. I'm pretty sure she thinks of me as loner (most do) and I don't want to creep her out.. We don't share the same circle of friends (mostly because I don't have any friends) but she came up to me and started talking to me during gym (so I can't be too weird, right?)

I found her on twitter but in a previous question I asked, I think it's best if I don't follow her until we become more acquainted. We're both girls and in high school. She's a senior and I'm a junior.

In short: how should I approach this girl I barely know and become her friend? Should I just stop her in the hallway? I don't want to scare her and catch her off guard. I don't want to embarrass myself in case she was only talking to me out of pity. (I'm quite the charity case).
Who's planning a friendship? All I want is help talking to this girl.