First of all congratulations for being interested in Steemit and crypto in general. You are well in front already.
Do you want to buy STEEM to Power Up?
First step to get some Bitcoin, although being 15 I doubt you will be able to use any AUD to BTC exchange but if you are in captital city you should be able to find a Bitcoin ATM or connect with a crypto Meetup group who will help you out.
RE: Is there any way I can invest in australia?