Find a photographer and have your head shot taken. The photo should be a representation of what you really look like. No glamour shots. Ask professionals (not your best friend and certainly not your mother) which photo is best. Get 8x10 prints of your photo made.
Compose a resume. List your name, age, hair and eye color, height and weight. List any experience you have -- including acting, dance and voice classes. Do not embellish or make things up. You are young, and agents won't expect you to have 30 films under your belt.
Get representation/an agent. Look in the "Ross Report," a performing arts resource found at Samuel French and other bookstores. Look for agencies that specialize in young adults. Compose a letter stating who you are and that you are interested in representation. Tell them you are including your head shot and resume for their consideration. Staple your resume to the back of your head shot and trim the edges off the resume so they are all 8x10 in size. Do not use 100 staples. Two will do the job - one at the top and one at the bottom. Mail the letter, head shot and resume in an 8x10 envelope (do not fold) and address it to a specific person. Do your research. Know the agent's name and address the agent as Ms. or Mr. Follow up in a week or so with a phone call or a postcard requesting a meeting (if the agent has not already contacted you).
Gain some theater and/or on-camera experience. Attend an open call at a community theater to gain experience. Attend open calls for union and nonunion television and film work. Get experience. Get experience. Get experience. Get some more experience.
Get a union card (SAG, AFTRA, AEA). This is a oftentimes a catch-22: to get a union card you need to be cast in a union project and to be considered for a union project, you need to already be in the union.
Most TV and film casting takes place in Los Angeles and New York, but many cities cast locally for commercials and films that are that are being shot in that area.
Research. Read up on acting. Watch movies. Watch good actors. Start to familiarize yourself with actors, directors, plays, movies, etc. If this is to be your career, you must take it very seriously.
Take acting, voice, and dance or movement classes.
Memorize some age-appropriate monologues that you like.
Get the support of your parents. When they see how dedicated you are, they will share your enthusiasm.