How can I have a conversation with a girl I like, but normally don't talk to.
You can start by saying hi, then smile at her, and eventually a conversation will become natural. Treat her like a friend so she won't be uncomfortable. :)
First off, I'm a girl! I want to establish that right now! Second, if you have the same class as her, ask for help on something. That or invite her into a conversation. Or, if she likes to rip on people, join in.
say hi and comment on something that she is doing. she will usually respond and you can go from there.