How do I become an actress

I have been wanting to be an actress for a while now but I'm not sure how to get there. I love the fact that when you watch/act in a movie you are in a completely different world and just the fact that you aren't yourself amazes me. I live in Sydney but not 100% sure how to and where to audition if I get to that point. Advice? What should I do to get on the big screen (I'm also 15 and Hoping this could be a career)