For Army ROTC, at least at my son's school, all you have to do is register for the MSI classes. Fall semester is Leadership & Personal Development, and Spring is Intro to Tactical Leadership - both 3 credit courses. You'll also register for PT (1 credit) both semesters. Although, as I understand it, not all schools give credit for ROTC classes. If your's does, you just need to make sure you leave room in your schedule for the additional 3-4 hours.
Again at my son's school, non-contracted freshman (i.e. non-scholarship) are able, for the most part, to participate in all ROTC activities - Ranger challenge, FTX, etc. They are not allowed to attend any sort of summer training such as Airborne, Air Assault, CULP, etc.