Look for an acting coach and sign up for acting classes. There are books and DVDs available to teach you the many styles of acting, such as the Stanislavsky method. Other great resources are "Breaking Into Acting for Dummies," by Larry Garrison and Wallace Wang; and the revised edition of "Acting for the Camera," by Tony Barr.
Take your acting and modeling abilities out on the town. Most communities have opportunities for residents to become involved in theater. Find out what's available and get an audition.
Have head-shot photographs taken and put together an acting resume. Include all your community theater experience. List any useful skills you have.
Set up appointments with talent agents accepting new clients. If you have a particular modeling or acting style, ask around at your acting class or theater group for recommendations
When your agent thinks you're ready, he or she will send you out for auditions. The book "Audition: Everything An Actor Needs to Know to Get the Part," by Michael Shurtleff and Bob Fosse, is a great resource that will leave you feeling much better prepared.