Enroll in acting classes. You need to learn the skills necessary to be an actor and a good one at that. If possible, take a variety of classes such as scene study, characterization, improv, cold reading, commercial and voice acting. The better trained you are the more likely you'll be to be called in by an agent and make a good impression.
Have some professional headshots taken. Make sure you have them taken in color and are prepared for the shoot. Take several different outfits and have all your grooming done beforehand. Don't go for glamor. You want a picture that looks like you on any given day. It should be representative of your character type (the acting character type you will be submitted for and play).
Create a resume on the computer. List your name, date of birth if under 18, any union affiliations you have, training, credits, education and special skills. Print your resume on a nice, professional looking paper. Cut the papers to fit the headshot. Staple a resume to the back of each headshot.
Look online for a list of legitimate agencies (SAG franchised agents). Read their information to see if they represent your type. If so, write a brief cover letter stating you are seeking representation. If you've won any special awards or have something you think sets you apart from other actors you may state that as well. Place the cover letter on top of a headshot and resume and mail it in a manila envelope.
Wait to receive calls from interested agents. They will set up a meeting time for you to come in. Be prepared to perform a monologue, commercial or short scene for them. They may also provide you with material to perform when you get there. Go into each interview and show off your skills and personality. If they are interested in you they'll let you know. Only sign if you are comfortable with the agents and think they can promote your career in a positive way. Good luck.