Get your status matched for starters, that may provide some benefits. Celebrity and Royal have some tier matching. E.g. Diamond equals Elite.
Depending on your status, there is laundry, happy hours, breakfasts, priorities and discounts.
Great that you are visiting our countries. You have plenty of time to check out the threads in this forum and also Ports of call forums, there is lots of information to glean through.
Perhaps provide your itinerary and cruise details as well as your interests.
You still need the Myki card, they are just put on on cruise days to make it easier and not interfere with normal commuters.
Cost can vary due to age and package. For me it was $16, which is loaded with $10 of transport use, which took us into the city and out to St Kilda as well.
Planning the trip and researching options and such is the basis for all the fun. Doing the trip and enjoying how the planning all comes together is the realisation of the hard work. Remembering is the cream on top.