We are pretty well-prepared for rotations; obviously you are not expected to do brain surgery (or anything else for that matter). But the biggest thing you can do to help yourself is read, read, read. After that, you can also rely on lectures/rounds etc to become more informed about what's going on. And hopefully, if you have a good attending/resident, you can ask them any questions you have.
Regarding choosing clinical websites, the best advice you can get is from fellow students who did the same rotation. They will tell u everything you need to know.