1. Use textbooks for revision (good for all subjects)
You know those thick textbooks which you use in class... Get them! Their elaborate descriptions and answers help you to understand those hard topics and help you structure high-level answers. If you can't afford them, go to your school library (they should have textbooks there) and borrow them.
2. Practice papers (good for all subjects)
Practice papers are a great way to test your knowledge and the more you do the better. They give you an idea of how your GCSE's might be structured.
3. Tutorials online (useful for maths and science)
Khan Academy, Hegarty Maths are all good websites for refreshing your memories or understanding new concepts. Take notes whilst you watching them for a better understanding.
Make a time table for your free time and studying time. You can't study all day or else you'll miss out on your friends and family. A timetable is like a balance for when to relax and when to study.
Find out how.