How can i become a movie,t.v show extra!?

So i really want to some extra work to get some experience to become an actor,get an acting Agent but i really want to know how but someone here told me to get a movie extra agent yeah but how cant i just go on a website or call please help oh and im 12 my parents are helping me and i live in burbank thanks to all who answer please give me at least a number or site Thanks!!