If I could offer some advice:
- Get to know your syllabus' (the syllabus is your friend!). After all, you can be examined on anything that it contains.
- Do HSC past papers (and trials or half yearly exams from other schools or previous years at your school if you can get your hands on them) and mark them, or have a tutor or Teacher mark them for you
- Work on your essays: make sure you have a good structure, plenty of examples and (for English) you know your techniques AND the effect they create (eg. the use of rhyme in "line" creates humour .... etc)
- Talk to your Teachers. Ask questions! If they see you are interested, they will be more willing to help. For example, they might proofread essays for you, or provide an extra tip or two, or give you some past trial papers they have
Lastly, good luck!