Qualifications
You need at least a four-year college degree. However, mo' degrees, mo' money, so consider getting the jump on your peers by getting a master's or even a PhD (source).
It will help if your degree is in something like criminology or psychology. That way you won't just know how to physically protect the plane, you'll also be able to read people and determine who could be a possible threat (source).
Previous experience in the military or law enforcement is definitely a plus in this line of work. With this background, you come to the job already knowing how to use a gun, how to protect yourself and others, and with quick assessment skills, ready and able to think and act quickly in tough situations.
You need to be in excellent physical and mental shape—sharp, smart, and strong enough to overpower anyone who threatens the safety of the passengers on that plane. You might sit on your butt all day on planes once you land the job, but you need to keep yourself in shape, hitting the gym in between flights.
No matter your background, you will be given over one hundred hours of intensive training in the use of weapons, martial arts, and hand to hand combat (source). Phew.