Learn how to produce music. Professional musicians use programs -- such as Cubase, Logic Studio and Reason -- to create music, along with live instruments. A successful composer for commercials must be a well versed producer.
Build up a portfolio. Composers who are new to the industry may wonder how to build a portfolio when they can't get any work. Work for free to get experience and gain contacts. Score commercials that already exist, by replacing the existing soundtrack with your own. This shows others that you have the talent and capability to score a commercial.
Market yourself. Build a website to show off the portfolio. Connect with production companies and advertisers, and advertise your services through traditional marketing channels.
Watch a number of different commercials and determine what type of soundtrack works best with different brands and themes. Learn the industry and understand it.