The very first thing that I did when deciding to go in the porn was hit the gym. A big dick can only get you so far. Your body doesn't have to be perfect but dammit it better be the best that you can get it at that time. You must devote yourself to fitness and discipline yourself in that way before you should even consider stepping on screen.
Secondly, you must decide on how far you want to take your career. You can either do a few videos and then drop out of the scene. You can stay in it for a few years before you fade out, however, within those few years you better make them damn good and really explode. Or you can start out strong in the first year to calm down a little bit and go slow and steady but always remain in the game. Eventually you can work yourself behind camera by wanting to know more behind the industry than the sex.