Specifications are on the exam board websites. Just type in your exam board and subject and it will most likely take you to it. If not ask your teacher, that's what they are teaching you from! As far as revision techniques:
Science- copy out the spec (it tells you exactly what you need to know) and past papers
Maths- past papers
English Language and Literature- Mr Bruff youtube
Languages- flashcard apps are fun
Geography- print of the spec and past papers
other stuff like mindmaps, recording yourself and listening to it and post it notes works for some people but I never tried it.
Look up youtube videos for some topics, some are really good.