Obtain an agent by showcasing your acting range with a demo reel. Finding an agent entails extensive research on which particular agencies accept unsolicited submissions. The demo reel is a visual resume that reveals any prior roles you have had, whether in TV, theater or film and is a key component to getting an agent's attention.
Sending your demo reel to multiple agencies is more likely to garner representation than just a head shot with an attached curriculum vitae. In the meantime, reading the trade papers, like "Variety" and "Backstage" will keep you informed of any open auditions so you can stay in constant practice and contact with fellow actors and important casting agents.
Call back to follow up with the agencies where you have sent resumes and demo reels. Once you have gotten an agent, bear in mind that it is a waste of both your time and your agent's to be sent out on auditions for roles that do not appeal to you or are not within your acting parameters.
Practice the lines for the audition until you feel secure that you have memorized the words and are able to add your own personal inflection and touch to the dialogue. The key is not to rehearse ad nauseam, but simply to get comfortable with the role you are about to embody.
Maintain your cool while in the waiting room at the audition. The worst thing you can do is psych yourself out for failure because of how intimidated you might be by other candidates.
Confidently and clearly deliver your lines to the casting agent. Cold readings tend to be a bit more intense because you are essentially delivering a monologue. Try to incorporate something memorable and unique into your interpretation of the role.
When your audition is over, wait to hear back from your agent to see if the casting director wants to see you for the next round of auditions. Though this can be a very stressful part of the acting game, it will all eventually be worth it when you gain your first major onscreen role.