How do I go about talking to him and what do I talk about?

Whatever you feel comfortable talking about, just strike up a conversation about that, but whatever you're going to talk prepare to bring the ammunition to carry the conversation to the point that its not 1: short or 2: filled with long silences.

Avoid anything too 'extreme' for politics, but, and this may just be me, even just small talk works, ask him how he's doing to start off, maybe what he did this weekend, or even ask him something about the material you're covering in class.

There's nothing that says you have to have a play by play script for how the conversation goes. I know, for me personally, that if a girl I don't talk to often strikes up a conversation with me out of the blue it catches my eye and I decide to see where its going. Because its out of the ordinary.

So don't be afraid to just talk to him, say hi, ask him some basic questions, things you'd talk about comfortably with friends, but keep an eye out for segways and tangents you can use, these are key. If he mentions something, like a hobby you share or something like that, pursue that topic.

However, if you're having trouble carrying the conversation it may be advisable to let it drop. Avoid awkward silences. If he wants to keep the back and forth going he'll fill the gaps, if not he's more likely to let the silences hang and roll off and do something else/go back to working etc.

Uhhh, critical thinking? Oh, wait. You look him in the eye and ask him about critical thinking. There. Another answer under my belt. This is easy!

Seriously, ask him about it. It will accomplish 2 things critical to your current life. #1 it will force you to study (you need to know this particular subject well in order to carry out #2)your previous class notes and help you get an A. #2 It will give you two something to talk about and possibly set up a study date. I have never had an easier time getting a date than asking for a study date. If you are the "learned" one in the subject, don't be afraid to offer your knowledge as a study partner. If he knows more about critical thinking than you, don't be afraid to ask for his help as a study partner. 9 times out of 10 when two people spend an inordinate amount of time together, they usually develop an attraction to each other. So, if you are not sure he likes you, I would bet that he would if you guys spent 3 or 5 extra hours a week together studying critical thinking together.

Pretty good huh? I still liked my first answer better. It didn't ramble like my last one did. :)