most of them require a picture/headshots. and be very, very wary of places that want you to take "classes", require that you use *their* photographer for headshots, etc, etc. there's a lot of scams out there. use the yellow pages, eye/now and don't be afraid to shop around or ask lots of questions.
if you can get on voucher that's much better than going as a regular joe-schmoe extra. they pay you more and feed you better food. by law, i think they usually have to feed you every four hours or so. actra's ideal but the stipulations of getting into *that* are almost impossible for a beginner to meet. you have a better chance of getting into actra if you're a woman or minority, and i think you need 6 speaking bits first.
if you get in, bring a change of clothes, lots of books, a discman and tons of cd's. for the most part you'll wait around for 8 hours and be on set for all of 5 minutes.
the money's decent...well, well above 6 bucks an hour, (forget how much i got the few times i did it, but once it was close to $15/hr,) and sometimes you get to meet cool people.
danny devito comes up to my waist, robin williams is short, shy and hairy, and jon stewart said i have moxy.
(ed norton's a bit of a dick, tho.)