travelling australia

[carolina]

hi guys,
i want to go travelling at the beginning, middle of july and travel from brissie up to cairns. Can anyone give me some tips or information what is really great to see? Where I should spend more time? I will have three weeks for this road.
After that I consider to fly from Cairns to Darwin. See the city and two NPs and than go with the train to Alice Springs and book a tour. Have anyone experiences with the train form Darwin to Alice Springs and can give me maybe also some information what kind of tours are really good in these area? I thought 17 will be enough time for this trip?
And than I would like to go to Western Australia. But here I do not have any idea what is great to see ... the problem is I am travelling alone and so renting a campervan is too expensive for me. I would be very glad to get some information what is great to see and how I could travel there bus, flights, train,??

I know a lot of questions but I still hope that some experienced, friendly people can help me

Thanks a lot! Carolina