Well, you have certainly jumped in with both feet; from a 3 day cruise 21 years ago to a 31 day cruise that most of us would consider the cruise of a lifetime.
What you should expect is an aboslutely fantastic vacation--no, adventure; from the minute you sail out under the Golden Gate to the minute you sail under the Sydney Harbor bridge.
Many of us here consider the Sun Class ships, of which the Dawn is one, to be the ideal size ship; not so big you feel crowded and exhausted from trying to get around; not so small you miss any of the many amenities we expect when cruising.
Your questions are very broad. I have been on Princess to Hawaii and from Auckland to Sydney so I would be happy to help with any specifics. All I saw of Australia was Melbourne and Sydney, from the dock to the airport, so I can't be of too much help there. But I'm sure there are many others here who have spent much time down under.
Have a great time planning this voyage and please come back with more questions.