I have the same problems of what to talk about, really with anyone.
I talk to this girl on AIM sometimes, she's in 3 of my classes. I don't like her, not like that anywhy. She's just a nice girl that sometimes attempts to talk to me or say hi, and I notice that she tends to look in my direction in nearly every class, though that could be in self defense.

Since I talk better when I'm not actually talking(like now), I've tried to talk to her several times on AIM, and she was a close freind of the girl I last liked and that big controversy(and she is in 2 of my classes). Outside of the controversy, I honestly can't talk to her about anything...I end up screwing it up. In Bio 2 days ago, we got a paper back that was maked oddly on everyone's, and she said "What's this 10/70?" and for some unknown reason to me, I laughed out loud. It wasn't a bad time to laugh, it was before class started. But she said "What's funny about that?" and I was screwed. I said "I...I got 0/70" and she's like "ohh...". So I was kinda disfunctioned for a while...there's been other times like that, and several times I tried to talk to her about it on IM, but I mess that up too.
It's like, unless I have a solid topic and reasoning, I'm screwed.
Which makes me pretty damn useless in conversations.
Eventually I'll be able to communicate with people in a practical way, but it seems every time I screw up now, I don't talk more.
And never assume a girl knows you like her. I tried, and it was a bad thing. That's another story...