As Luvepoker say, there are site sponsored pros, and to become one of them you typically need to be well known in the poker community first, which mean you must already have some kind of poker related career. There is not much point for sites in hiring some totally unknown person to promote the site. But maybe what you actually mean is how to get a backer or become part of a stable? This is something completely different, and here the answer is, you typically need to create a track record first, so you can prove to the backer, you are worth investing in.