How to become an Extra in Melbourne

Hi everyone, I'm in year 10 and I really want to get some work as an Extra in either tv shows, commericals or films in Melbourne. I think that being an Extra would be a good start as it may give me a sense of the atmosphere in the acting field...? Anyway, does anyone have some advice on how to become an Extra? I've read so many posts, read all the advice about agencies and everything, but I still don't know how to become one! Help me please! Thankyou so much