First, you should join an extra's casting agency. An obvious one to join is Central Casting, located at 220 S Flower St. in Burbank. I can't remember the number, but 411 will know. It's good to call first, because there are specific days to join. It costs $20 to join, but with a lifetime membership.
Another good place to join is Hollywood O.S., which are located at 400 Beverly Drive South. Not 400 Beverly Blvd, not 400 Beverly Dr. 400 Beverly Drive South. I just say it because I got lost when I went to sign up with them. They're open Monday through Friday till about 5:00p. Hollywood O.S. is a place that holds about five or six casting agencies, specifically Sandi Alessi (the ones who've cast Pirates and Mansion). It'll cost you anywhere from $20-$40 to join, depending on how many agencies you want to sign up with. Also they sell a great book, Extra Work for Brain Surgeons , which has some great information you should know for extra work (legalities, etc).
Those should get you started. You'll hear about more from extras on the sets. You'll also hear about call-in services, which is what I use. If you want to book yourself, just join an agency. If you want others to book you (for a monthly fee), get a call-in service. Many times, you can get more work with a call-in service, except maybe when it's slow. A few call-in services are Cameo, Extra's Management and Networks. You'll get information on those when you sign up for an agency.
Extra work is a good job for you if: you like to sit around, you like free food, you like to see famous people and you like flexibility. Extra work is not the job for you if: you like to be treated with respect, you don't do well in the cold or the heat, you like making more than $1000 a month on a regular basis (even if you're union)... actually, this list could go on. It really could. There can be a lot of cons to extra work. I've been doing it to get my SAG vouchers. I have two, I need three. Many people get them in a few months or a year, but it's taken me a little longer. But I'm not bitter