Qualifications
Some employers require just a high school diploma or equivalency, although it's not like a college education would hurt you (source). Certainly not as much as a shiv to the abdomen.However, most employers require a bachelors degree, full-time experience as a counselor, a prior job in law enforcement or the military, or some combination of these (source).
Regardless, you need to be physically and mentally tough and know how to wield a 'cuff. Not surprisingly, many prison guards come from military parents.