Try to think about what has worked best for you in the past, different people need different preparation for exams. For example, many people just focus on memorizing the theory as much as they can by rereading or repeating it in their minds, as well generally spending a lot of time rehearsing the books.
But it took me a while to realize that strategy didn't work for me at all (I'm bad at memorizing), and that what worked for me was jumping straight to the practical part (for physics, mathematics, etc) and only reading the theory when I'm stuck. As for theory-only subjects, what helped me the most was writing a story or summary of the contents in my own words, as if I was writing a novel. This is just me though, it doesn't work for everyone.
So try to find out which method is best for you, there's not much else to say,
Dil covered the rest well.
Oh and a final tip, when I start exams I try to write an outline of the main ideas (for history, literature, etc) or formulas (physics) that I'll need throughout it. It's usually useful for me as it helps me clear my mind and not second-guess myself in the middle of the exam.