I would recommend finding a domme in your area who would be open to teaching you - an 'apprenticeship', if you will.

Many girls learn from the "bottom" up...

(Sorry, couldn't resist)

Seriously though, quite a few dominatrixes learn the biz by first "paying their dues" as submissives in situations where the (male) client is looking for a domme+male sub+female sub scenario.  Or as being the sub in lezdom porn.  Etc...

The idea is that you have to have experienced some of this from the sub side in order to have an appreciation for "pleasure-ful" pain vs "painful" pain.

Many of the best dommes out there learned by "interning" in this way.  Some girls never leave the intern stage and become pro-subs, some switch, and some find that they prefer to always be in the dominant role.

So Yeah, I'd say find a highly regarded dominatrix in your area, and try to become an "apprentice" of sorts.  Also, find the fetish/leather/whatever groups in your area--you'll mostly find either lifestylers or wanna-bes there, but you might find one or two people in groups like that who really know their stuff and don't mind sharing some of that knowledge with you.

Good luck.

Please see my previous thread answering the same question.  I'm a member of the lifestyle kink community here in L.A., and I'm glad to point you to regional resources if you want to PM me here with your city/location.  FYI, BDSM is illegal in some states.

In addition to the resources mentioned in the link, I would add one more . . .  the Masocast podcast has in-depth interviews with dozens and dozens of well-known professional Dommes, and many of those interviews address topics like "How did you get started?

then join groups.

Recommend Dungeon Servitus, Dungeon V, and club x.  Attend their workshops.