The 3 main examining boards in England are OCR, AQA and Edexcel. These are monitored by the JCQ (Joint Council for Qualifications).
As a student or parent you needn't worry about which board you are enrolled with, your school will do this for you. There are often similarities between the topics, and the assessment objectives that each examining body set (due to criteria set by the Department for Education) though the exams will be slightly different and in content and form.
Follow the links to find all of the course materials for each subject via the examining bodies online portal:
Edexcel subjects - Pick a subject, then download the course specification, past papers, teaching materials.
AQA subjects - Pick the 'GCSE' from the menu then explore available subjects and download course specifications and past papers.
OCR subjects - Pick your subjects from the list to download the course specifications, past papers and mark schemes.
If you have any other questions related to course materials, specifications, past papers then don't hesitate to get in contact. We have a wealth of experience and resources that we are happy to share with you.