Copyright the script. Submitting it to the Writer's Guild of America registry for copyright is the most common way to accomplish this.
Write a logline. A logline is a short description of the script, usually two or three sentences. The logline should be catchy and make people want to know more about the project.
Write a summary. The summary is a one- to four-page narrative of the script. Write it as if it were a short story, not an outline. Write actively and avoid going into too much detail about things that don't concern the primary plot.
Learn the market. Figure out which production companies deal with subject matter similar to your script. Contact these companies and ask if they accept unsolicited material. Make a list of the companies that do, along with their addresses. The "Hollywood Creative Directory" is an excellent source for finding contact information on production companies.
Make a list of agents. Pick up a copy of "Hollywood Agents and Managers Directory" and search for agents who deal with screenwriters.
Write query letters. Send them to your list of production companies and agents. Individualize them with the contact's name so it doesn't read too much like a form letter. Contact a few dozen at a time, and include a SASE so they can contact you back without too much trouble. After a week, send another batch to a few dozen more.
Have patience and persistence. The television business is extremely hard to get in to, especially if you don't know anyone in the industry. You may have to contact a hundred agents and production companies before you even get a nibble, and that may not even pan out.
Join peer-review websites such as Triggerstreet to have your script looked at by other writers who are either in the business or trying to get in the business. Constantly rework your script based on ideas and reviews by the people who read your work.