Learn everything on the course spec - no more no less.
Do tons of past papers (make sure you time yourself!)
Read the examiners reports.
Depending on what you're studying use sites like:
bbc.co.uk/bitesize (notes, videos and more)
studywise.co.uk/gcse-revision (neatly organised revision resources- you can find course specs etc here)
CGP revision guides are the best - get ones tailored to your exam board for each subject.
Hope helps!