I assume the business side of porn is no different to the acting business. At the start you do small time filming's, like local shoots/amateur. After that you get an agent who then gets you contracts, interviews, and shoots etc. You run over the contract with your agent and see if you agree to the terms and what you have to do. You then go for an interview or rehearsal where the director will check you out (naked of course) to see if you suit the part. You then shoot the film. I'm sure it's easier said than done of course but I'm guessing that's the jist of it.