How do I start talking to this guy on Facebook?

see if he puts up an interesting status and then send him a message asking him about it, or leave a comment about it and see if he responds to the comment. Or a simple way would be to ask him something about the class to start and then build off of that.

Talk about the class, since that's what you guys have in common. What he thinks about homework, tests, the teacher, etc. After that you can just casually ask him what he likes to do for fun, and he should get the hint that you like him. But it's always easiest starting out talking about something you have in common. Makes everything less awkward. Good luck. : )

Be careful that you're not two different people on fb and in real life. I find I am more confident on fb than in real life with girls. So I can talk to them on fb but be really shy in real life. So be careful in that respect.

Just be your self if he accepted you then he is probablly some what interrested in you. If I were you I would give a few days to see if he sends you anything.