How can I become an extra or background for movies or television?

I am a teen (16) in high school and I wanted to know what it is I have to do to work as an extra for movies or television. I have no experience what so ever. Do I need a resume? A head shot? Are there places that I should apply to that will hold my information? Who should I approach? Or can I do this all on my own? I have no idea how any of this works, so any details would great, even some sort of step by step process, haha. Would I need an adult around the set since I'm under 18? I live sort of close to LA, but unfortunately in this little boring town. I don't know if my location would make a difference.