I think a lot of us were scared about beginning rotations. You feel like your comprehension is poor, your memory is worse, and you are scared that you are going to screw every task that they throw at you. Let me reassure you.
1) Every good student feels this way. Actually, if you're worried about your performance, odds are favorable that you're a better student than most. The bad ones, and a lot of the average ones, don't really care at all.
2) The reason you haven't retained the majority of the factoids from medical school is that the majority of the factoids you learn either got dropped from your memory because they were useless (biochemisty, half of path) or were building blocks for future knowledge that are not easily recalled in isolation (pharm, anatomy, physiology, and the other half of path). You're just about to start learning the stuff you're supposed to retain.
3) Rotations aren't hard, because you don't have any real responsibilities. They suck, but they're not hard. If you are like most medical students you will rapidly progress from being scared that you are going screw everything up to furiously wondering why the f- you're there at all when they don't have anything for you to do. Honestly I don't think there is any way to fail if you pass the shelf exams and show up on time, and if you made it through the first two years you can pass the standardized exams.
Anyway the point is that you should relax: you will be fine. If you have more than a week between the step and rotations and you're really freaked out you can try to read through the case files for your first rotation so you feel like you have a few factoids to fling at your residents, but mostly I would recommend you kick back, relax, and wait for the suck to start.