Obtain professional-quality photo head shots. Find a photographer with experience in that area to do the job. Perform in showcases, student films and any parts you can land. Create a reel of your work -- the more roles portrayed on it, the more presentable it is to a casting agent.
Obtain a directory of casting agents and casting directors, as well as a directory of TV-production companies. The Hollywood Creative Directories are an established source that provides this type of information, including contact info for producers and writers of television shows. Professional, lead-acting job tryouts are primarily secured by talent agents or managers.
Send your head shots and resume to casting agents and casting directors.
Register for one of the many Internet services that track auditions, such as Actorspages, Spotlight, Auditions TV or Auditions Auditions. Submit your head shots, with contact information, as required by each site.
Read the industry trade magazines for actors, such as Backstage East or West if you are located on either coast, near New York or Los Angeles. Also, check Craigslist classifieds for acting work, local classifieds for open auditions and subscribe to government activity site emails for casting calls. It isn't necessary to live in New York or Los Angeles to find open local auditions, especially if registered with an Internet site.