What Will I Need to Revise?

So, I have my GCSEs coming up in the next few months, so I just wanna know what I should revise, and what I shouldn't waste my time revising.
E.g. For Chemistry, should I or should I not revise bonds.

So, the exams that I am taking are;
Higher Maths
Unit 1+2 Sciences (Chemistry, Physics and Biology)
English Literature (The History Boys, Lord of the Flies, To Kill a Mockingbird)
English Language
Music Theory (12 Set Pieces)
I.C.T
Media Studies (Crime Drama and Magazines)

Out of the subjects above, what topics should I revise in each of them?
Thank you.