how do you start a conversation?

You must greet her first and introduce yourself and show her a smile then you can be making a good conversation afterwards because people who are smiling (not if it has dirts) are mostly friendly so people will easily be comfortable to talk with you.

I'm a girl but I know what you mean. When I want to talk to someone I just say, hi, how are you?? I might comment on something they are doing . Maybe reading a book or listening to music. Just ask a question about that. Its that simple. If they want to talk to you they will.

The best advice I can give you is think about whats best to say to her (considering you know her better then we do). For instance, I like a girl in history class. I would start just greeting her in the hallway (more then once, otherwise she might be think "who is he?" If she likes you, then she will want to talk to you), A simple "Hi" would do the trick. Then now that she knows who you are, I would try and spark up a conversion with her. Dont think about talking to her, Just DO IT. They say the hardest line to write is the first. Once you get that out of the way, it will get easier. Like stated before If the girl likes you, the conversation will continue, if she doesn't, don't worry theres alot people on this planet. No matter how ugly, or how good looking you are, someone is bound to like you. Lastly, don't be afraid of rejection. How are you supposed to find someone who likes you, if the ones that dont like you, wont reject you? I hope this helps. Good Luck.