The "official" tourism organzation for the Cairns Region is:

Tourism Tropical North Queensland
51 The Esplanade      (between McDonalds and the Casino)
Cairns Qld 4870
Tel (61) 7 4051 3588
Website: http://www.ttnq.org.au/

Before you get to Cairns, also check out these:

www.australia.com for a good overview of what Australia has to offer.
www.queenslandholidays.com.au for general Queensland information.