Choose which university to attend. Different universities have different ROTC options. For example, the University of Washington has all three programs: Army, Navy (and Marine Corps) and Air Force. Other schools will have only one or two programs: Indiana University has Army and Air Force ROTC detachments. Still other schools, such as Harvard, have no ROTC programs --- the nearest host university (or cross-town affiliate) for all three programs is the Massachusetts Institute of Technology (MIT). This means that Harvard ROTC cadets go to MIT for ROTC classes, although at the time of writing in May 2011, Navy ROTC is expected to return to Harvard by the end of 2011.
Decide what you want to do as a military officer. All services need specialist officers in such fields as law enforcement, finance, supply, intelligence and medicine. However, different services focus on different types of combat. Only the Navy has submarines and only the Air Force has bombers. If you want to be an infantryman, both the Army and the Marine Corps have infantry. If you want to practice medicine with the Marines, join the Navy because the Navy provides all the medical (and chaplain) services for the Marine Corps.
Decide which service you want to join. The services have very different personalities and images, but this will vary greatly by unit and by specialty. For example, Navy fighter pilots and Navy submarine officers are all Navy officers, but there are vast differences between submarine and flight squadron culture because the work is so different.
Visit ROTC programs you're interested in attending. Talk to the cadre (officers and non-commissioned officers who will be training you) about the reality of their lives in the military. If you're considering the military as a career, you should be asking about work-life balance, especially if you are female, but also if you are male.
Decide at leisure. Joining the military is like marrying; it's one of the most important things you will ever do. In the end, what service you join should be right for you, not your parents, friends or anyone else.