To find out what you have to study you first need to familiarise yourself with the syllabus. To download the syllabus click here
The syllabus is usually written in a very formal way so you might have to ask your teacher to “translate”. For AQA exams you can find for each section a syllabus in student speak which will detail what you need to know in a language that you can understand on this website.
The secret to good grades is not only to acquire the knowledge but also to practice exam papers so you become familiar with the expectations of your exam board. Follow the links to get past papers. Read the marking schemes. The examiners’ comments are also a mine of information, they tell you what was expected and what mistakes students made, so read them and don’t repeat the same mistakes make new ones! To download past exam papers click here