There are many ways an artist can break into comics, but the most important thing to remember is that you don't automatically become a good comic book artist by simply being a good, even great artist. There is a whole array of skills an artist must develop to be able to do good comics, the most important of which is storytelling.

To learn how an artist can apply his talent to comics, one must understand how comics work. There are no two better books out there to teach that than these two books:

UNDERSTANDING COMICS by Scott McCloud
http://www.amazon.com/Understanding-Comics-Invisible-Scott-Mccloud/dp/006097625X

COMICS and SEQUENTIAL ART by Will Eisner
http://www.amazon.com/Comics-Sequential-Eisner-Instructional-Books/dp/0393331261

Once you feel that you are good enough to do comics professionally, visit the websites of the companies you want to apply to like Marvel or DC, and then look at their submission guidelines. All of these websites have some form of submission guidelines in some form or the other. Follow these carefully. DC comics no longer accepts submissions through the mail, and only accepts them at Comic Book Conventions.

Conventions are another great way of showing editors your work for evaluation. Search online for a convention happening near you and attend it.

Create a presence online. Upload your work at a website and then join networking sites like Comicspace and DeviantArt, then join comics forums and participate. Who knows, your future employer might be lurking there. Be careful of anyone you meet online who wants to work with you. Google their name first to check their credentials and reputation. It's always best to be careful.

I've written an article like this for my blog, but geared mostly for breaking into Philippine comics. Check it out here:
How to Get Into Comics Today
http://alanguilan.com/sanpablo/2006/01/how-to-get-into-comics-today.html

Good luck!