Get a head shot. A head shot is a professional photograph which typically reflects you from the shoulders up. Any time that you go to an audition, you need to have at least two head shots in hand. Hire a professional for this; they know how to draw the look out of you that directors are looking for.
Create a resume. Even if you don't have experience on screen or stage yet, create a page that includes identifying information, contact information and what interests you. Include anything that will cause an agent or director to want to get to know you better.
Do an audition. The single most important thing you can do to get yourself on screen or stage is to get yourself in front of the directors who are deciding on the actors who will perform. This is done in an audition. This is your chance to show them what you have. Even if you do not think a part is right for you, or you do not think you will like it, go to the audition. The very process of auditioning is important to your career development. If you do not have an agent, you can locate auditions in major newspapers and on websites such as 800casting.com.
Join a union. Once you have some experience under your belt, you will have an opportunity to join the Screen Actors Guild. Acting is a job, and as such, it involves contracts and pay negotiation. The Screen Actors Guild can provide you with resources that will assist you in this area. There are some acting jobs that you will not be eligible for until you are a member of the union.
Get an agent. An agent gets paid a commission to find you work. They do not get paid unless they do, so it is in their best interest to get you working. You do not have to be experienced in order to get an agent. Many agents will host open calls, a time in which they will look at just about anybody desiring to get on stage or in front of a camera. Don't expect an answer immediately though, they will likely just "call you later" if they are interested in working with you.