I'll just throw into the discussion before it starts that you really do have until after ATARs come out to decide on what to do. If you're unsure of your preferences, just put something down in August (especially before September when timely round applications close off), and you can (with the exception of most of November, they close off change of preferences for a month or so around then).
There's the exception of some courses that might have an early cut-off date for when you can enter them into your preferences lists, so another reason why it's worth looking at courses and doing the initial application early. Usually it's the courses that have a range of criteria like folios, interviews, other tests and so on that might choose to do that.
You don't have to fill out all 12 preferences (IIRC you can get away with having 1 preference, but you'll get a message recommending that you should put in more preferences), but I'd say it's probably a good idea to have at least a few options in mind. Personally I filled up all slots on my VTAC and UAC applications just for the sake of completeness, but there's no compelling reason to go out of your way to pick 12 courses.
You can find out all the dates and stuff from the VTAC website, I won't give a direct link since it's worth the time reading through all the information on that website for yourself. If you're interested in interstate unis, then you can find the relevant authority for each state from here
http://www.actac.edu.au/