Qualifications
Anyone who tells you all it takes is natural ability and dedication is lying to you. The ballet world doesn't give a hoot about your uncanny ability to stick a Hortensia (that's an actual move) unless you've had more training than a nuclear physicist has.
Seriously—a professional ballerina trains for more years than it takes to get a PhD in nuclear physics. Don't be fooled by the makeup and the pretty costumes. These girls know their stuff. No one "picks up" ballet like a game of street basketball. Did you notice that none of the dancers in Step Up seem to be classically trained in ballet?
To join their ranks, you'll need to start ballet—oh, let's say, six years ago. You definitely need to be training one-on-one with an esteemed dance instructor by the time you're entering middle school. (And, by the way, you probably need to come from a family with enough money to afford the expenses—those lessons aren't cheap.)
Dance through high school and attend a college with a top dance program. Don't bother with anything less than the best.
Then, when you've been dancing for the first two decades of your life, go get your MFA (Masters in Fine Arts). Not technically required, but an MFA is a smart move, especially if you think you'll one day teach spoiled brats how to dance once you start losing your spot to younger, better dancers.