For preparation before the hsc year I probably wouldn't start writing essays right away but I think what you should do is find out all your prescribed texts for the hsc, and what topic your class is going to start with. After that you should try to sort through a few related texts (if your class is doing discovery or mod c first), and choose one, and read or watch your related and prescribed and try to set up some basic document with quotes, technique and analysis, that you can later use to build an essay after you have learnt more about how to structure the essay for the specific topic. You might want to also start writing a creative (probably will be terrible at the start, but you need a base story written to improve it later on), because from my experience my school didn't spend much class time on creatives compared to other modules.
In terms of study during the hsc, what worked for me was to write an essay over a few days after a few weeks of learning about the texts in class, and get that first draft marked by a teacher or tutor, and continually improve on it until you have reached the exam, where I would memorise the essay and adapt it on the day to the exam question. After receiving marks and feedback after each exam you should continually edit and make the essay better, even if you get a good mark, because in the hsc you're competing against not only your class but the entire nsw.
