1. Work up a decent portfolio
Make sure that you have a portfolio that can convince an agent of your knowledge of lighting posing an composition. You don't have to have " text book" models in your portfolio to get the job. Models associated with agencies are usually only allowed to shoot with photographers that are approved by their agents. To gain experience ask good looking people in your network if they be willing to shoot with you. Have images taken in studio and outside as well. It is important to show to the agencies that you are capable to work in and outside of studio.
2. Locate agencies near you
Find agencies near you, go to their website, study their style and find out who is the person in charge for male or female models.
3. Contact the agent
Short and sweet. This is your first round. It will be a relatively short conversation unless you are a well known photographer. Introduce yourself and ask if you could email your website/ portfolio. Email your link to your website and give it a few days. If you don't hear from them it does not mean that they did not like your work.
4. Follow up
If you don't hear from them after 3-5 days call them and ask if they had a chance to review your portfolio. There is a good chance that they were just busy with fashion shows, castings or recruiting new faces. It happened to me more than once that an agent reviewed my portfolio with me over the phone. If they like your work they will be sending you new faces for test shoots and there is how you get one foot in the door. If they don't give you an opportunity right away it is quite alright. Ask for feedback and how can you further improve yourself and try again.
5. Test shoot
When you first work with model agencies you are most likely will be photographing new faces to contribute into their portfolios. Portfolio shoots are usually expected to be clean and simple focusing on the models' physical attributes and their ability to showcase different expressions.
6. Build relationships with agents
Agents, depending on their models' book will ask for certain shots. Whether they need more indoor, outdoor, commercial, more skin or casual, I always shoot for the agency first. After that is done I ask the model if he wants me to shoot anything particular and/ or I just sneak in a few shots with a new kind of lighting I am experimenting with and show it to the agent anyway.
7. Get Paid
Agencies normally want you to do a couple of free test shoots but after that if they are pleased with your work you could absolutely initiate a discussion about being compensated for your time and talent.