Cross-dressing, Trapping

Well, that depends. If you're interested in posing on someone else's site you just need to go through that site's channels until you get to the right person. Alternatively, you could find yourself an agent (yes, even pornstars have them) and let them do the leg work for you...but they'll take a cut.

If you want to start your own site, the first person you need to talk to is a lawyer. You're going into business, which means you'll need to get a tax ID number, incorporate (I'd suggest an LLC, it offers a degree of legal separation between your actions and those of your business,) and make sure all of the proper papers that prove you're a consenting adult are on file.

Hope that helped.