Exam Boards
There are 3 in England and another for Wales. Typically they provide the relevant specifications, guidance, materials and information about support and training. There are opportunities to sign up for aditional materials, including papers and mark schemes but you will need to use a school email address and know your centre name and number (exams are a big business).
It is essential to know which board your school uses especially if you teach at KS4. However you may want to look quickly at free resources from each of the boards.
Boards have increasingly linked with publishers for textbooks, so check if your school has any linked textbooks that are still relevant.
There are two tiers at GCSE, Higher and Foundation which brings a problem of deciding which of these to enter for borderline candidates (and in some schools this could be a large group of pupils).
AQA
Your schools may already be signed up to Exampro which contains thousands of questions, mark schemes and examiner comments. AQA Maths Home page
OCR
Has a useful plan your curriculum scheme of work builder. OCR Maths home page
Pearson Edexcel
Includes a very useful Mathematics need to know formula sheet. Pearson Edexcel Maths home page
