Qualifications
A lifetime of going to see every movie you can get your eyeballs on (including those weird "art" films). Rule number one: obviously, you have to be someone who loves movies.
You don't have to go to some fancy film school, but it sure helps to go to college and be a writing or film major—especially a good college with a good writing or film program.
While even a film major at Podunk State knows his Einstein from his Eisenstein, some schools have better film criticism programs than others and will also allow you to form friendships and connections (or, as the biz calls it, networking) that'll help you later in life when you need a job.
Plus, graduating from a fancy film school will look wicked sweet on your résumé when job-hunting. Don't just watch movies; learn the pieces that make up a film. Is it good, is it bad, and how do you know? Pick apart every scene, camera angle, and possible subtext of everything you see until you can no longer just go to a movie for enjoyment; you do it because you know more about film than everyone else and can tell them why.
That doesn't sound pretentious at all...
We can't stress this enough, though: this is a writing job. You'll have to hone your skills as a writer if you want to hack it. Take lots of writing and journalism classes in college. Learn how to command the language and develop a style. And if you can't think of the right pop culture reference to pull from your encyclopedic knowledge of film, there's always Google.