Make an appointment with a photographer you trust, and ask them to take color headshots of you. Be prepared to pay between $150 and $600 for your photos and ask that they make at least one original of each picture you are planning to use. You should have one friendly looking photo for your commercial headshot, and a more serious looking photo for your legit headshot.
Print out a resume created in Microsoft Word or your favorite word processing program. Your resume should include your theater, film, and TV experience, your education, and any special skills you may have. A school or church play also counts as an experience, so if that's your only credit, put it on your resume and feel proud of it. If you have ever been in a school play, put this on your resume as well. Staple your resume to the back of your headshot.
Move to New York or Los Angeles. If you are trying to start an acting career outside of New York or L.A., it will be very difficult to build up your resume. The best, easiest way to become an actress is to move to a suburb near one of these two cities and audition as much as possible.
Look at trade newspapers like Backstage, especially if you live in Los Angeles or New York. This way you will be up on the new show auditions, new agents looking for talent, and new classes that are available in your community. Send out your headshot and resume to any audition you are interested in that is listed in Backstage, even if the character description doesn't exactly fit. They may be interested in your headshot and call you in anyway.
Go to open calls. The fastest, easiest way to get discovered is to attend an open call for a film, musical, or play. Most Broadway productions have open calls once a year as per contract, although they rarely cast anyone. You are not guaranteed to be seen unless you are a member of Actor's Equity Association, but if you show up early enough, you may get your big break. Look for open call notices in Backstage and on Playbill Jobs (listed in the "Resources" section below.)
Buy a copy of this year's Ross Reports. The Ross Reports provide an extensive list of every reputable talent agency in New York and Los Angeles, and can be purchased online or in bookstores in New York and L.A. Highlight every agent you'd be interested in working with, and send them a headshot and resume as well as a cover letter describing a little bit about yourself and why you should be hired.
Prepare two contrasting monologues. Go to the library and read plays. Do not choose a passage from a musical, even if it is spoken. If another character is speaking, remove their lines from the passage. Your monologue should total two minutes of spoken text. You can also find monologue books in the library with ready-to-go pieces, but you can guarantee your auditor will have already heard the same monologue that day from someone else. Be prepared to recite these monologues from memory at auditions and for agent appointments.
Take acting classes while auditioning to bulk up on your acting chops. Do not let your skills slide, even if you are busy auditioning. Take dance and music classes, even if it isn't your strong suit, to match your competition's strengths. You should be taking at least two classes a week in order to stay in both physical and mental shape.