These are the most important resources!
Each course has its recommended textbook or series of textbooks. Teachers may use these in class, but often won’t allow them out of the school for fear that they’ll get lost or damaged. If this is true, then, for the sake of around £17 – £20, I suggest you buy one. They are your main resource and explain everything in detail with worked examples and lots of practice questions.
There are usually two or three written for each course by different publishers. It makes sense to buy the one that your teacher uses in class. To make sure you get the right one, get the ISBN number from a class copy and use this to search on Amazon.co.uk.