Try BTCmarkets or Coinspot as initial. Coinspot especially is very easy to use (but rates may not be as good). Both allow payments in AUD via Poli payments which is run by AU Post.
Only put in what you can afford to lose.
If you invest a significant sum down the track do not keep your coins on the exchange but download to a hardware or paper wallet.
Invest a small sum and move it around and get used to the terminology before investing too much.
