OMFG LEARN TO SPELL. Jokes. (One thing about this site, never take anyone or anything seriously and you'll love it here)
On topic: Depends on the subject.
For maths and large bits of physics, you need to have a solid understanding of the topic. The best way is to continually attempt questions, not just the same type of questions, but different ones too.
For for content heavy subs, just grab as many textbooks and summarise with reference to syllabus dot points. Cut all the irrelevant shit out, to stop wasting time.
Though tbh most of doing well in exams is how you actually apply the content and thus learning these skills (e.g. writing eloquently) should be your priority right now.
And lol this is really far off for you, relax and enjoy while you can.