Take a variety of headshots. Have a family member take a photo with a digital camera that can be blown up to 8 x 10 is acceptable. But if this is the first step of your career as an actor, then you might as well spend the money and have some professional shots taken. It may cost you more upfront, but in the long run you'll appear as a professional, ready to go.
Surf the Web. Being an extra on TV somewhat location specific, meaning you need to find the shows that shoot in your general area. New York, Los Angeles, Louisiana, North Carolina, the city of Chicago, Connecticut, Rhode Island, Florida and Texas all have a significant amount of televisions shows that shoot in their general area. And if you live in Canada, almost every province has an active film office. Plug your nearest major city into a search engine with the words "casting office" and "extra" into it. Note the results.
Contact the casting office. In the major cities, many casting offices do not accept general phone calls--in the major cities contact "central casting." That's where you'll submit your headshot and casting information. (A lot of this is done via the Web, so depending on the office you may just be able to scan it in and fill out a form.) For the rest of America and most of Canada, they are still operating on the old school "submit a headshot." So gather their contact info, mail off your headshot, and follow up with a phone call to be sure that it arrived and didn't mysteriously get "lost."
Stay current. If you move or change contact information you must repeat the previous process. Casting Directors move too quickly to waste time to try and track you down. They'll simply go on to the next person who is your type. Be sure they have the right contact info.
Arrive early and be polite. Being an extra on a TV show is fun, but it can also be a little boring. So be sure to bring a good book, your PDA, video games you can play silently and some snacks. (You will probably be fed a meal, but depending on the size of the production it may not be that good.) Also, when the casting director calls you, they will probably ask you to bring wardrobe. Don't bring your whole closet! Simply a couple of choices for each "look." If they want something very specific, they'll provide it.
Do as you're told. Don't start asking the director if you can say a couple of lines. It's against union rules unless they upgrade you (which does happen on rare occasions if you are on the verge of joining the Screen Actors Guild). The person who will be in charge of you is the second or third assistant director. They are the captain of your ship.
Cash the check and watch yourself on TV. One of the cool things about being an extra is you usually get paid. Not a ton, but between 50 and 150 bucks a day for your time. Many older people make extra money on the side by doing extra work on TV and are still able to collect their pension and other benefits.