After your initial success in cs394n, we usually continue the research with a number of cs395 follow-ups. These are independent study courses where the idea is simply to get academic credit for working on research. In the beginning, you usually take one cs395 along with your breadth or depth classes; when you are closer to your proposal, you may take three cs395 classes and concentrate only on research. These classes are pass/fail, and you get a pass if you make reasonable progress. We set up goals in the beginning of the semester and revise them as we go along. If you have never taken a cs395 or cs394n with me before, I ask you to write up a paper on your research at the end of the semester. If you get significant results, we might even turn that paper into a conference submission.
It is perfectly ok to explore different topics while you are doing cs395 courses, and even different advisors. The idea is to get an idea what research is like in this area, and which topics would be fun to turn into a dissertation. There is no particular hurry to decide on a topic during your first two years here. Explore and have fun. After that, you should have a pretty good idea of what you would like to do, and it should not be too hard to narrow it down to a dissertation topic.