What are you interested in? What do you find yourself liking most about Computer Science? Pick something you have a bit of knowledge about coming into it, and write about that.

Or... alternatively, take a topic you know very little about, focus on that, and then write your thesis based on that topic. That way, you'll come out ahead both ways-- you'll have a wider knowledge base on a topic, and you could end up with a good grade.

Have you thought of anything yet yourself? What does interest you about Computer Science? Let us know what you've thought of, and we'll help you pick something out, or maybe advise so you can decide on your own.