Go to a school that teaches aerobatics!

I don't know of any though.

Everyone's right so far... need your PPL first, although you could take an aerobatic lesson any ol' time.
As far as a career goes (airshow or competition performer), one thing that all of those folks have in common is a tremendous amount of hours logged doing aerobatics. It's a lot of fun (I have 1 hr. of dual in a Zlin 242 logged), but it's hard work, and after getting your body used to it, you need to do it regularly, like an athlete staying in training.  But there are all sorts of levels of shows and competitions; you could probably start showing off almost right away, once you get the hang of the basics. Finding sponsors who want to pay you to go nuts in an Extra, Sukhoi, or whatever- there's probably a different story for every sponsored pilot out there.You're not going to see a job like that in the classified ads.
And don't even think of trying to finance a high-level aerobatic plane on your own, unless you're filthy rich. They all have sponsors, even the ones who have "regular" jobs on the side (airline pilot, etc.).
  If you just want to do it for fun, ignore all I've just said and find an instructor with an aerobatic plane for rent- he or she will answer any other questions you might have. Go for it!!!

The outfit I had my advanced aerobatics lesson with does a complete course on it. (Extra in my Avatar.) I seriously considered taking it up but decided against it. This would involve getting my PPL first as my licences expired long ago. When I enquired they can take you right through from basic training to your advanced display licence. This would obviously cost plenty but I could afford to do it if I wished. If I was younger things might be different. For one thing I'm getting older & not as fit as I used to be, You need to be extremely fit to do advanced aerobatics. http://ultimatehigh.productionarea.com/academy/index.php

Another way would be to get your PPL & purchase an aerobatic aircraft of your own. Then go & practice. This is the route Denny Dobson took. http://www.dennydobson.34sp.com/team.html

thanks guys for all the input