Well i wouldn't just go up to her and tell her you like her because if she doesn't feel the same way then you'll feel bad about it and what not. So I'm thinking that maybe you could start by increasing your conversations to something more than small talk and try talking to her on myspace or im or w/e you may use, then after sometime ask for her number and go from there. Slowly build up to the point of telling her how you feel don't rush it. =)

be cute, interesting and confident and just talk to her and ask her out. be in control and dont be shy because she might be simply waiting for you