I am currently in the process of joining the ADF (Assessment Day May 13). I have finished year 12 last year and have looked into becoming an Officer.
Firstly, you do need to be smart. They don't let just anyone become a leader. You will need to do an aptitude test (on your JOEs day) which is compulsory if you want to be anything from a cook to a mechanical engineer. It is similar to an IQ test. I personally did very well on this test and have the aptitude to become an Officer, however there are other factors.
Secondly, you will almost definitely not get in straight after high school. You can apply but it is competitive and most people who go to RMC (officer school, basically) are between the ages of 20 - 25, as 18yos do not have enough 'life experience'. For this reason I have chosen to omit an Officer application and spend a few years in the ADF and then apply.
Thirdly, you need decent grades in Year 12 (at least straight Cs) and you need to be a confident person.
Long post, but I'll provide good answers to your questions. Tank crew in the ADF is generally accepted as boring to be honest. You don't get deployed and you will be posted to Darwin barracks which is apparently a dump. However, ASLAV crew (light armoured vehicle) get deployed often.
My advice to you is to apply through General Entry to either Main Battle Tank crewman or ASLAV crewman. Then after a few years apply for Officer.
EDIT: Whoops, didn't see that you had already applied, lol. Well if you can't pass the aptitude test it's bad luck for now as aptitude is generally something that is given to you at birth or strengthened in early childhood. If I were you I'd apply General Entry as mentioned and make your way up to Warrant Officer.
lol how stupid can you be.
I've done that test and it's easy as piss.
If you can't pass that then please don't become an officer.
Don't be an asshole.