I do chemistry & biology, and am currently in year 13. For chemistry, I would suggest develop a solid understanding of the basic concepts. Try watching someone called 'Eliot Rintoul' on YouTube, who does videos on all AQA topics. For biology, I would suggest to, not only conceptualise topics, but also regularly go over flashcards / notes to ensure key biological ideas stick in your mind. Closer to exams, perhaps try using sites like 'physicsandmathstutor' to develop exam technique. I wish you the very best of luck in your studies!