There are a huge number of subjects offered to HSC students. They range from typical English and Mathematics subjects to Vocational Education and Training. A full list can be found here:

http://www.boardofstudies.nsw.edu.au/syllabus_hsc/course-descriptions/

HSC subjects are generally taken during the HSC year (year 12). However, some schools offer the opportunity to accelerate subjects in earlier years. This is not uncommon, though it is usually extended only to extremely talented students. Certain subjects, such as English, cannot be accelerated, but others, especially mathematics and languages, are commonly undertaken by students in younger years to ‘get them out of the way’. However, most students sit all of their HSC subjects during their final year.

Many subjects have extension courses available. English, Mathematics, History, Languages and Music all have extension levels, which are challenging courses that build upon the material taught in the non-extension courses. Mathematics, for example, offers two extension options, each one harder than the last. Extension 2 Maths is among the most challenging school mathematics courses offered in the world.

What subjects you choose is dependant on your personal preferences and the subjects that are offered by your school. Not all subjects are offered at all schools, though there are options available for students with a real passion for certain subjects, such as Open High School.

Hope this helps!