I'm Miranda, and I'll be happy to help you with your question.
I do have some past experience in the modeling industry (independent photography and some independent modeling), so I can definitely give you some advice about how to become a fitness model.
I'm assuming you're already a muscular, well-built guy, so the first thing to do is build up your modeling portfolio. You need to have some professional headshots (head and upper torso) taken. I would also suggest that you have some full-body photos taken as well. Headshots can be very expensive, but there are some photographers out there who are willing to take free headshots for you in exchange for you allowing them to use those photos in their own photography portfolio. This is something I did when I was first building up my photography/modeling portfolios, and I received some amazing photos out of it.
Some places that you might find photographers who are willing to do this are Model Mayhem (http://www.modelmayhem.com/), and you may even find a reputable photographer on Craigslist. If you do go this route to get headshots, bring someone with you to the photo shoot. Never go alone. Ever. There are a lot of crazy people out there, and it's better to be safe than sorry.
Model Mayhem is also a good place to start an online portfolio, as many photographers, models, etc., can see your photos and might want to work with you.
Once you have some good photos, I would suggest looking at the modeling agencies in your area and attending an open casting call. Most modeling agencies have multiple casting calls throughout the year to find new models. However, make sure that you research any modeling agency you are considering working with. Are there any scams associated with them? Good reviews? Bad reviews? Are they in good standing with the Better Business Bureau? There are a lot of modeling agency scams out there, so research and be careful.
You may also be able to schedule an appointment with the modeling agency directly. Just search their websites for policies on appointments, potential new clients, etc.
After all of this, if you get signed with an agency, they will be able to help you further your career as a fitness model, which may eventually lead to you being sponsored by a supplement company. But it's a long road with a lot of hard work to get to that point. Modeling is difficult and must be taken seriously.
Also, whenever you are on a photo shoot (independent or agency) or anything else, make sure to be friendly and professional. Introduce yourself to everyone you can and ask for tips. It's a great way to make contacts, and you never know who you'll meet.
I hope that answered your question. If you have further questions, please ask. It's been my pleasure helping you. Please remember to rate my answer positively, as rating me is the only way I receive payment/credit for my work. If you would like to request my help in the future, please post your question with "For Miranda Only" at the beginning, and I will respond.