Well if your sig is true, that you are in Yr 10...
A very common regret is subject choice. You will make yr 11 and more so yr 12 less stressful for yourself if you make the RIGHT subject decisions for YOU. And it's very personal, there's no clear cut answer.
Another common regret is not doing more/enough work.
Leaving things to the last minute.
Letting the HSC take over your life, sacrificing too many things just makes you bitter and sad. Some people's friendships decay to competition, it's really sad.