Becoming a professional comic-book writer is an incredibly challenging goal. Artists have far more ways of getting recognized and can have very long careers, but writers tend to be phased out after a few years, and generally get paid far less than artists, which contributes to the difficulty of making a career out of their work. There are exceptions to the rule, such as Mark Millar and Grant Morrison, but many of the great comic writers are also artists, like Frank Miller and Todd McFarlane. Because the industry is very difficult to break into if you cannot draw, you'll need all the help you can to get your foot in the door.
Write a script. Don't wait for everything else to fall into place; just write your script and use it as a way to show people you're serious. There isn't an industry standard for comic scripts, unless a particular publisher has a favorite format, but the scripts do need to contain certain information. This includes the number of panels on each page, descriptions of each panel, and the dialogue and text for each. Although some professionals prefer more general story outlines, most seem to write full scripts, so familiarizing yourself with script writing is necessary.
Network at comic conventions. Conventions are a great way to meet industry professionals, as well as other aspiring talents. Professionals will have booths where you can meet and talk, and will also be walking around and mingling. Don't be afraid of mingling and meeting new people, but try not to be pushy or overzealous. These professionals are willing to discuss the industry, but will not read your scripts and don't have time to talk only to you. The more people you know and the more who know you're a writer, the better your chances of getting an opportunity to write. (You can find a calendar of conventions in Resources.)
Submit samples to comic-book companies. All of the comic companies have guidelines on how to submit writing samples. Some, like Marvel and Dark Horse, are looking for writers to submit original works. In this case, go to their official websites and find out their submission procedures. However, other companies, like Image, are only looking for creator-owned material, so they do not accept writing samples. In this case, you can find an artist you work well with and try to make your own comic to sell to a company. Sometimes, online comic-book forums can be a meeting place for eager artists and writers who are looking for an ideal partnership. Whether you're submitting a writing sample or creating a comic first, make sure to follow the various publishers' submission guidelines, which can be located on their websites.