Theresa, there are a variety of visa types, depending on how long you want to stay and if you want to work here etc. you can apply online through the Dept of Immigration: http://www.immi.gov.au/visitors/tourist/visa-options.htm
But assuming you're coming for a short stopover on a cruise ship, if you want to get off the ship, yes you'll need a visa. The e-visitor visa sounds the best. All the guidelines and forms are at the above web address. Your travel provider may even do it for you.
Best safety tip for Australia - watch out for the drop bears. No, it's a joke, they don't exist. Really.
Phil from the safety hub about 5 years ago