Make a decision. Do you really want to do this and are you ready for the commitment it requires? You have to really want it or else it just isn't worth it. Start taking classes and training in your discipline. You need to have some skill before you set out to start in the industry otherwise you will be eaten alive and have all your self-confidence stripped of you.
Get some great headshots taken. Your picture is what will get you get an interview and your foot in the door. If you have a bad picture, there will be no interview. Put together a resume. It should list your education, training, experience and special skills along with your contact information. For modeling also list your sizes/measurements and height/weight. For singers have a demo tape/CD made.
Find guides online and in print editions. Guides such as the Ross Reports and Talent Managers Guide can be ordered by phone or online as well as found in specialty bookstores such as Samuel French. Read through the books and online guides and highlight the managers who represent your type. Some guides make note of who is currently seeking submissions and who isn't. Take note of these details as well.
Create your list of those you plan on submitting to. Staple resumes to the back of your headshots. Two staples on top and one on the bottom. Type a simple cover letter to put with each headshot. Keep the cover letter quick and to the point. Place your headshot/resume and cover letter in a large manilla envelope and mail all these submissions out.
Wait to hear from interested managers and set up meeting dates. Get to your meetings early, cleanly dressed and ready to show them your skills. Good luck.