You need a small ego, or else ballet will cut it down to nothing.
You need a teacher who's good at engaging the mind. Traditionally, ballet has been taught to children by rote. That's not good for adults, and adults don't have that kind of time either. You need to use your adult mind to advantage. Ballet technique is a beautiful theory, like mathematics, that can be built from first principles. Find a teacher who will elucidate that.
You need to learn as much as you can about how your body works, and how you can develop it. For example, according to some sports medicine research, anyone can develop a split in 2 months, using the "most efficient" stretching techniques. It took me a lot longer, as it does many other people; we typically spend a lot of time trying things that ultimately aren't very useful. The more you know about your body, the better off you'll be.
You need clothing that shows your whole body, that doesn't fall down, and that you feel comfortable in. If footed tights keep falling down at the crotch, cut off their feet and roll them up to just below the knees! If you're not too shy about it, wear a contrasting-color leotard under your tights and do away with the T-shirt; you can see so much more.
You need patience.
As for dancing in professional ballet shows: that is actually not out of the question. As for a full-time professional ballet career, get real: the only ballet jobs that pay anything close to a living wage are with companies you have NO chance of getting into. So whatever you profession is now, you want to keep it.
My story: started dancing at 5, always liked it, never "professionally" serious. Stopped for 5 years at 21 (that's when your body changes and you get all sorts of injuries until you learn more about your body). Began pursuit of a professional career at 26. Now 29, dancing as apprentice with a professional ballet company (that's the track to join the professional company, if I continue to progress as I have been). My part-time "day job" is database development for a financial firm, I'm living more comoftably then most everyone else in my ballet company. So you see, a professional career is not out of the question; but you won't know how far you can go unless you try.
--- Bob[/i]