Anyways depends what you are looking for in Australia as to where is the best place for you to visit.
If you are looking for "outback" go to The Nothern Territory. There is Ayers Rock etc out there, youve probably heard of the whole "dingo got my baby, azaria chamberlain tale" That happened out there. (but dont let that turn you off).
If you are looking for "city" go to Sydney it has the Opera House, The Harbour Bridge, the harbour and the 2000 olympic park. Theres an aquariam, a zoo and a big IMAX theatre. You can climb the Harbour Bridge (one of our icons) there are tours etc, some beaches.
If you are looking for "homey" go to tasmania (though it is detached from the main australia so you will need to fly/boat it down to there. Tasmania have the tasmian devils there in the zoos, they are going extinct and found no where else in the world..they are good to see (vicious though) Along with Port Arthur (a historical site..lovely gardens)
If you are looking for "beauty & fun" go to The Gold Coast in Queensland, spectacular beaches, lovely warm weather and you can go scuba diving over the Great Barrier Reef, the Largest and ofcourse most beautiful reef in the word. They also have Brisvegas, in the city of brisbane (in Queensland) It has the whole "las vegas" theme. This is also the main area of Amusement parks in Australia. They have Sea World, Movie World, Wet and Wild and Dreamworld.
It really depends what you want to see as to where you should go..but i reccomend going to all those places (if your budget allows) because it really is a great way to see Australia...to experience all aspects. But by the sounds of it Queensland looks like the best for you with the reef, the amusement parks and the beaches..