FIRST, get shots done by a professional photograher. Check your local "yellow pages" for photo's and see if they do "modeling-type" shoots, specifically something like athletes, but not totally necessary. You will want head shots, workout wear, and some "normal" clothes. You want to do posed and action shots. Mimic the shots you see in workout magazines.
Now, you will do one of three things with the photos:
1) Send them to supplement companies and/or stores. If they like your look, they could put you on contract or on a shoot-by-shoot basis. Include a cover letter.
2) Send them to an agent. Not required, not necessary, but if you use an agent, THEY do the leg work.
BUT, you also give them a cut of the money, and there is no guarantee.
3) Send them to magazines, again, with a cover letter. Send them to ANY mag you can think of that includes "fitness" spreads. These might not pay, but you will get exposure. Send them to the "fitness editor" or something similar.
Now wait, and wait and wait.