Hi, I have experience with this as a person who has scholarships and has been involved in open days at UNSW for scholarships. All the scholarships I know about offered to high school students going to university you can only apply for the scholarships once you are going to be a high school leaver (i.e. in year 12). Usually applications open mid way through your HSC and closes around September to November depending upon the university you are applying for.
However, the scholarships offered each year are mostly the same and therefore even as a student in lower years in high school you can still research about them and the types of scholarships offered. If you know what type of scholarship you are looking for such as one that is all round based, leadership based, academic based (these are often the hardest to achieve as they have high ATAR requirements which are usually listed most of the time) or sports or equity scholarships, you can begin to plan what types of skills you need to acquire or demonstrate in your application.
The best applications are the ones that show the person is well rounded and has participated in a wide range of activities rather than just academics. Despite this though that doesn't mean you should try to do every single possible activity to place on the application, rather if you have a few that you devote a large amount of time to and are gaining valuable experience in them that is much better than smaller activities just for the sake of an application.
So what I would recommend is since it says in your bio that you are a 2020 HSC student don't worry about applying for anything, but focus on looking at which scholarships you would be interested in and try to work towards having the characteristics they have outlined in the selection criteria.