Balance your acting studies with your schoolwork. Since there are laws in place to ensure young actors don't skip their education while they're making movies, it's important that you don't let your acting career interfere with school. Many young actors go to special performing arts schools for a balanced education. This may or may not work for you, depending upon finances and where you live. There's the Tara School of performing Arts in Colorado and Long Island High School for the Arts. Performing arts high schools specialize in integrating the study of the arts with traditional curriculum. Consider looking into one of these programs if you're serious about pursuing an acting career.
Build a portfolio. In today's world, a portfolio is more than a folder with a few 8-by-10 glossy shots and a resume. If you can sing, include a demo on CD of you performing a couple of songs. If you can dance, a DVD of you performing is a good addition to your portfolio. Include a DVD that showcases your acting talent as well. This could be a recording of any theater you've done, even if it's only a high school play. If you don't have any credits under your belt, have someone film you doing a short performance to demonstrate your ability to act. This multimedia portfolio is what you will use to try and secure an agent.
Find an agent. Agents can get you into auditions and even get you jobs based on your acting resume. You pay your agent a percent of the money he makes for you, but the money is well spent. A good agent has contacts. You can find agents by looking through an online database and searching by state to find an agent who will represent you (see "Resources"). This is where your portfolio will come in handy. An agent will want to see your resume, 8-by-10 headshots and probably see you in action before they consider signing you.
Take as many acting jobs as you can, even if you're offered commercials. You may want to do movies right away, but everybody starts somewhere. Consider talking to your parents about moving to one of the big film cities. California, New York, Atlanta and Chicago are hotspots. You don't necessarily need to be in one of these locations to make it in the film business, but living near the entertainment capitals can be a huge benefit.