To OP, CONGRATS!!
what you ABSOLUTELY MUST understand however is the following (don't think i'm being mean, just giving you the hard facts so you don't accidentally embarrass yourself
You will not get "discovered." This is a misconception some people have from the old Studio System days in Los Angeles. In those days, the industry was a very different animal from what it is today
Extras work is just that...WORK. Expect to be seriously worn out by the time you're done. Each scene is shot from several different angles and is shot many, MANY times until the director is happy (helpful hint: the phrase "checking the gate" means that they like the way it looked on the monitor and they are making sure there wasn't anything wrong with the camera, i.e. probably done with that scene)
DO NOT talk to the actors. If they talk to you that's fine but keep the conversation to a minimum as it is a general rule not to talk to them in case they are in character, which can ruin them for a scene.
DO NOT look at the camera or think that it is looking at you. You can hope that you will be "seen" but most of the time the depth of focus they are using is so narrow that anything further than 3-5 feet behind the actors is just a blur
If you want the wardrobe department to love you, really try and follow their requests when casting sends your call time. Unless they have a uniform for you to wear they'll usually ask you to bring clothes of a certain "look" (ex. "New York in the spring" and give a list of specific articles of clothing). Bring lots of options but ALSO try and compile an outfit to wear that fits their description. If you get good enough you'll never wait in line at wardrobe again (if they like what you're wearing they'll just OK you)
Bring a book or something to occupy yourself with. We have a saying in the extras community: "Hurry up and wait." Extras are generally treated like cattle. You will be there, depending on the complexity of the scene and how many angles the director wants to shoot from, for at least 10-14 hours.
If you bring an ipod or something expensive like that, keep it on you and do not leave it in Extras Holding (the place they keep you until they need you).
If you cannot resist taking pictures, leave your phone in your glovebox. Nevermind the IF
I get caught bull, you WILL get caught and you WILL NOT get asked back. It's one of the very VERY few things that can actually get you "blacklisted" per-say. Depending on the paperwork they have you sign you can even get sued if you take and distribute images of the production.
If you are on a health-specific diet then bring your own food that doesn't need to be refrigerated. Usually the healthiest thing, at least down here, that they provide is a crappy tossed salad.
If you want to have a good time of it, make friends with the CREW. They are the ones you will work with on a consistent basis. Everybody down here may not know my name but most know me as "Speed Racer" because of how i drive. PAs (production assistants) are your friends and will be your controllers usually. They'll place you and tell you what to do. Occasionally the director himself will give you your "Action" (what you are "doing") if you are close enough to the camera or doing something noticeable enough to warrant more specific direction.
All-in-all try to have fun. Lots of people aren't cut out for it. They think that they're owed some sort of respect but in truth, you are cattle. You literally can be replaced at the drop of the hat so any kind of drama is a very bad thing. Many go in thinking its gonna be a ball and getting makeup and meeting actors and even getting "noticed" and becoming famous with a reality television series (i know people like this) but they never go back. But if you can handle the long/bizarre hours and having to do the same thing over and over again for hours at a time (before being told to do something else for the next few hours) then you'll have a blast!