The ATAR is a rank (not a mark) which is used to work out how eligible you are for university courses.
If you knew (and you don’t until the day before ATARs are released) your final HSC marks you can calculate your ATAR fairly accurately. The atarcalculator site does a good job of that.
Keep in mind that if you are in year 12, you won’t have your final marks available to you until they are released and then the next day the ATAR is released. So not having the final marks makes it very tricky to calculate your ATAR beforehand despite what anyone tells you. On the other hand if you are likely to get a very high mark because you are bright and study hard then you know that you are likely to be in the top band 6, so estimating that you will be in the higher ATAR range makes sense. It’s when you aren’t confident of your actual HSC results that estimating your ATAR is notoriously difficult.
So when you apply for uni (you have up to 9 preferences) make sure you apply for courses that span a wide range of ATARs.