Traveling to Australia, need some suggestions.

Hello, I'm a 16 year old swede who is travelling to Australia with my dad for the first time. We are arriving in the middle of December and plan to stay in Sydney for some days (but we will come back so we can celebrate New Years eve there) After that we plan to roadtrip to the Great Barrier Reef and dive.

We have a total of three weeks in your country and besides our short stay in Sydney and diving plans in the GBR we don't really know what we should do. I would really appreciate if we could get some tips on what to do, both in Sydney and also on our way up to the GBR.

Btw, I've read that Cairns is the place to stay when you are visiting the Great Barrier Reef, what do you guys think about that?

I really look forward to visiting your beautiful country!