Train as an actor. You may hear stories about rank beginners making it big, and sometimes it does happen. But you'll improve your chances dramatically by being better prepared.
Develop poise. Contrary to what you might expect, you don't have to be glamorous or perfect to land a job in commercials---commercials look for all types. It is important to be comfortable and expressive with your body.
Practice for the camera. Record yourself and study the footage carefully to correct any problems you find in your presentation. Make certain that your speaking voice is articulate and pleasant to listen to.
Create a photo package. It's a good idea to have a primary head shot that accurately presents your true appearance and personality, as well as a few shots showcasing your range in portraying characters. Note that photographers' fees vary widely. You don't have to go with the most expensive service available, but you do need excellent quality, which almost certainly means using an experienced photographer. A head shot is a powerful marketing tool that can get your foot in the door. It is no longer necessary to have massive quantities of these photos printed, as many submissions are now done online.
Build a resume and portfolio of experience. For a commercial actor, this will include a video reel. Get whatever experience you can on camera, even if it means volunteering to do a promotion for a car dealership on a local TV station.
Make certain you live in an area with a viable market. Even in this age of Internet communication, you won't find nearly as many opportunities in Kansas City as you will in New York, Los Angeles or Chicago.
Get an agent. Admittedly, it's easier said than done; in fact, obtaining an agent can be as difficult as getting work once you have one. But without an agent, it's virtually impossible to succeed in the competitive world of commercials and film.
Regularly scour the trade periodicals such as Back Stage for audition opportunities. Don't expect your agent to do all the work!