Take commercial acting classes. There is an art to learning how to work in front of a camera, including how to slate (saying your name in front of the camera, and, if you are a minor, your age, as well as showing profiles of your face) and read copy.
Have a professional headshot taken by a credible photographer. Blow up the best commercial shot and have prints made.
Create and print your acting resume. Staple a resume to the back of each headshot. Submit to agents looking for new commercial talent.
Interview with agents until you find one interested in you that you are comfortable with. Sign with them and add their logo to your resume.
Go on your commercial auditions and perform well enough to get a callback. Then nail your callback to book the job.