To get more than 90% in CBSE12th is not a difficult task. You need to be through with the subjects. Sometime english do some unexpectetion. So try to be good in english too. Practise for all your subjects and revise on your regular basis. Have clear concept about the things. Try to write neatly and in order in exam. Plan a routine and implement in your study. And you can get 90%+ .
All the best!