First up, know why you're going, is it a holiday, or do you want to work?
If you're doing a holiday, then make sure you book up as much as you can, you don't want any empty days, seriously. You can explore most of the cities in a single day by foot so you'll soon get bored of wandering around if you've got too many days to kill. Book everything from the UK, it gets very expensive very quickly out here, but you'll get a good deal and be able to plan it all nicely with the tour operator in the UK. You need to know what you want to do, but it's all pretty much the same stuff that everyone else does. Bungee Jump and Skydive, Snorkeling at the great barrier reef, Fraser Island and Whitsunday, Sydney attractions, Byron Bay, Bondi Beach... etc just work out what you can afford and do as much as you can. There's about 20 different options and easily 100 operators all selling the same shit, so just go with what your operator suggests. Remember it's a holiday, so make the most of it.
If you're planning to work here then it's a whole different story. I'd recommend not doing any of the suggested intro packages (they're usually pretty shit), but instead book yourself a week or two at a reasonable hostel and prepare for some boring days... Try to fly in on a sunday or monday, so you've got the whole working week to get yourself sorted. There's a lot of stuff you need to get prepared:
Phone - head to Vodaphone, or any of the other shops and get the cheapest you can find
Bank Account - easy enough, make sure you have your phone first though! You can't get paid until you have an aussie bank account.
Tax number - this stops you paying too much tax, there's about 1000 ways to get this, so it shouldn't be too hard.
RSA - This is required if you want bar work, and is a bit pricey, but get it asap.
RCG - This allows you to work somewhere that has gambling, which a lot of bars do, so it's pretty useful
Other qualifications - depending on what sector you want to work in, there's probably a certificate needed, it's a ballache, but if you arrived at a good time you can get it sorted pretty fast.
Once all that is sorted you can start the job hunt, this is also a ballache but remember, the aussie minimum wage is around $20 an hour, so the price of a beer doesn't seem to so bad then! You can save the hassle of getting certificates if you go for farm jobs, or fundraising. Farm jobs are usually miles away, but offer free accomodation and if you do them for 3 months you qualify for a second working visa.
Bear in mind that Australia is expensive! Cheapest beer I've seen is $5 which wasn't even a full pint, and a Big Mac meal is $7, which works out at about £5. So it's expensive, until you start working, then it really seems quite cheap. But when you first get out here, you'll be draining your bank account a lot quicker than you might think - so avoid the expensive stuff until the end of your trip.
Finally, whether on holiday or working, take a friend! It's pretty scary moving cities in the UK, but when you consider that you'll have no-one to go to the pub with, no permanent place to live and no job it's really easy to get homesick. If you are alone, then stick with it, you do meet people and make friends pretty quick, but when you're sleeping in a room full of strangers and running out of money it's pretty lonely.
