Enjoy it obviously. I don't know what school you're from so I can't say how hard you'll have to work for 98+ ATAR but work just enough and then enjoy the rest. Study hard play hard. Don't burn out, don't overstress too much year 12 isn't that much harder. Take photo's throughout the year, trust me. Everyone will tell you it will go by faster than you can imagine, this is true. It won't feel like it day to day, but one day you'll wake up and half yearly's will be over. The next day trials will be over and the next day you're giving advice to someone in year 11 on BOS. Don't fuck up half yearly's and trials. Look into cadetships/scholarships etc in the summer holidays. Look at the requirements and try to get on top of it early, it'll save a lot of time for when you should be studying for trials/hsc.
In the end just make sure you enjoy yourself and don't worry too much.