I kind of semi-lazed around. Got decent marks, top 3 is almost all subjects. *cough* English. So just work hard and if you are capable which I think you are, that should be enough.
If your school has an assessments book then read it. It should tell you how much each assessment task weighs. To be ranked at the top you basically have to study & give it your best shot.
It's relative. In maths, I have to do lots of work to be top three but I could sleep in some subjects and be top three. Obviously if your school rank is higher it would be harder to be top 3 in everything and if it lower, easier.