Use the Screen Actor Guild's "agent associations" page to find reputable agents you might want to work with. Research each agent to find out what kinds of actors they represent. Make a list of agents you think might be a good fit for you.
Call or check each agency's website to find out their submission guidelines for new talent. Usually, you will need to send the agency a photograph of yourself and a resume listing your relevant acting training, performance experience, special skills and contact information. You should also include a letter requesting an audition.
If an agent grants you an audition, show up in a clean, attractive outfit that suits your age and personality. Be prepared to perform a monologue, read selections of scripts called "sides" and be interviewed.
If you haven't heard from the agency within six weeks of your initial submission or audition, call them to follow-up. Don't badger the agency or call them repeatedly, unless you are specifically asked to call again.
Once you sign a contract with an agent, he assumes the responsibility of submitting you for roles suited for your type and talent. If you come across a role you want to audition for, ask your agent to submit your resume and photograph to the casting director.
Whether or not you have an agent, you can attend open calls -- auditions open to the general public. Check listings for open calls in actors' resources like Backstage. Be prepared to wait in line for hours with dozens or even hundreds of other actors before you're seen by the casting director, director or producer.
Whether or not you get a callback or book the job, send a brief "thank you" note to the casting director, producer and assistants who conducted the audition. In addition to being courteous, a brief note helps directors remember you for future roles.
Network as much as possible. Attend industry events and conferences like those hosted by actor's unions to meet movie directors, casting directors, fellow actors and other professionals who can help you land roles in the future.
Enroll in a casting director workshop, a day of classes taught by casting directors that prepares you for auditions. In addition to learning what casting directors like and don't like at auditions, these workshops help casting directors remember you. If they like how you perform in a workshop they may call you in for auditions in the future.