There's a lot of good advice here about studying consistently, focusing on the syllabus etc. I have a feeling (but check) that examiners' comments on past exam papers are published. Reading those helps you see what sorts of answers are rewarded with marks, and what sorts of answers score fewer marks. That can be useful in helping to focus your study and learning how to answer exam questions.

Another thing I haven't seen here yet: enjoy yourself! I'm guessing that at least some of your subjects are chosen because you find them interesting or have some level of enthusiasm for them. So enjoy the opportunity to learn about them. Having that sort of attitude focused on big-picture interest could help you get through the more tedious (but necessary) sections of your courses.

Friendship is important. Don't go crazy with the partying, but have fun with your mates. For me - and most people I know - the year group really bonded through year 12 and a lot of the petty nonsense, bullying etc fell away as people realised that they're just a few months away from adulthood in the big wide world. Sure, you might not see many of your high school friends once you finish, but the good times make the year easier.

Final bit of advice. There's very little in most HSC subjects that's actually difficult. A lot of it is just a matter of memorising stuff - be it quotes for English, formulas for physics, ways of answering types of problems for maths, topic-relevant vocabulary and grammar for languages etc - and being able to apply that memorised knowledge to new situations and questions. And memorising is usually just a matter of time. Performance in the HSC does depend to some degree on intelligence. But it depends a whole lot more on discipline, organisation and just putting in the time to get stuff done.

Hope this helps. As always, your mileage may vary!