If you want to get around 98 should you start studying in year 10 or year 11. not sure :S
Year 10 - get some year 11 books or in maths get a copy of 1st year uni Calculus text (James Stewart or Thomas & Finney or whoever) and look at them as a matter of curiosity. If some topics strike your fancy explore further into it. Before long you'll have a great deal of additional knowledge, at a higher level, not necessarily perfect or correct. Hopefully this will fire up an interest and develop a skill in self-study. Soon you'll have a lot of tentative knowledge of Prelim Yr work. You go on from there. With the head start you won't need to do foolish and harmful all-nighters like many HSCers are now doing. You will be ahead and confident and relaxed. No "oo I'm stressing, boo I'm going to be screwed in my trials etc" for you. Don't listen to those saying:" don't worry about prelim yr work; relax until you begin HSC work."
When reading new and more advanced material you may at first be completely lost. Don't worry. If, at first you do not succeed, try and try again - as the saying goes.
I take it you want to do well right?