I'll keep my response general here, but at the school I went to before ruse it was mostly straight A's.
In terms of ECs I was part of a very good band (outside of school), was in a few sport competitions (nothing particularly high level), and school clubs/interschool debating, etc. Don't worry...
I can't remember if there even is a 4th section, but none of it was particularly difficult. If I were to guess I'd say maybe 3 sections were english (writing, reading comprehension, and grammar?)
I was in your position last year applying to join ruse in year 11 and I'm now at ruse, so I can give you a few pointers.
Not sure if this year will remain the same (as with all the advice I'll give here), but there are 4 test sections, 1 for math, 2 for english, and another that was fairly...