Instructions
Build up a body of published work. Most of the time this means publishing in places that pay little or nothing, like neighborhood papers, supermarket circulars, school papers or any venue that will print your work. This tells publishers that your work is good enough to be published.
Hit the comics convention circuit. The best shows are the San Diego Comicon every summer, the Chicago Comicon over the July 4 weekend and the Wondercon every April in Oakland, California. Publishers have booths set up and will often assign editors to look at the portfolios of aspiring cartoonists. This way you can make a personal connection.
Have a professional resume with clips of your published work. Mail (don't email) these to every comics publishing company you can find, especially prior to each convention that you can attend. This will increase the chance that your name will already be known when you actually attend the convention and introduce yourself in person.
Scour the Internet for places such as Lulu that allow cartoonists to have their work produced in a print format only when a customer places an order, reducing overhead and inventory. Another valuable option is CafePress. Artists can sell merchandise that have the logo and artwork from their creations.