I did biology, chemistry and maths.
Maths - Exam solutions will become your best friend. Just leave enough time before your exams to watch all the videos :')
Practice practice practice.
Chemistry - Read up on what you will be covering in class so that you aren't completely lost and can get on with the work in class without too many problems. Also chemguide is really useful.
Biology - biology relies heavily on memory so just sit down and make notes, and then make them again in a condensed version, make flow charts, spider diagrams, record audios of you reciting processes.
For all of these subjects, the only way to pass your exams is by doing every single past paper that is out there.