as above, and you can also find your nearest amateur theatre group and get involved. if you tell me where you live specifically, i might be able to give you some recommendations.
ATYP (Australian Theatre for Young People) also run workshops and classes. I think it's something like $40 to join as a member, last time I checked. (someone correct me if i'm wrong) If I lived anywhere near it, I'd join up myself. The couple of workshops I've done there through school have been really fun.
Since you're still at school, if you're at a public school you can audition for the state drama company and it's related things (namely the drama camp) and see where it gets you.
Do drama as a school subject if you're not already doing it, obviously. And just keep your eye out for opportunities to get involved in theatre/film in your local area and any short courses or workshops being offered by acting schools. and if you are serious about acting as a career rather than a hobby you really need to work at it, audition for as much as you can, take as many classes as you can, get together all the professional paraphernalia ie headshots etc, sign up with a talent agency, bla bla bla.
there are a couple of members on this forum that as far as i know are quite knowledgeable/talented in this area - you could try asking alleycat or skillo for advice, i don't know of any others off the top of my head.
good luck
