Hi Dipali!

Scoring 90+ in board exams seems to be more tough than what it actually is, you just have to focus on clearing your basics about any topic and be prepared to solve a lot of questions either from previous years or sample question papers. Most importantly, NCERT is the book which you should rely upon. It is my advise to firstly complete the Ncert textbooks in PCM at least three times because most of the questions are framed directly or with some minor changes from Ncert textbooks. Also, you can refer Ncert Exemplars and Arihant Sample Papers for these subjects. Since you haven't specified full details in your question, I have assumed that you are asking about Cbse class 12 boards. If your query was different, then please furnish the full details and we would surely help you.

Hope this helped.