Well, if you are looking forward to get an admit into a good and reputed NIT, you need to strengthen your basics about Physics, Maths and Chemistry, you can refer to the books issued by the central government of India for classes 11 and 12 and you need to score a percentile above 90 to get what you desire.

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https://www.askiitians.com/iit-jee/how-to-prepare-for-iit-jee.html

Hello ayushi
Well, I'm sure you know that the biggest exam for engineering in India is JEE mains and JEE advance.To prepare for these exams,the best way is to take a coaching for these.You should start preparing right after class 10 and be thorough with PCM.It's not that tough but you need to work hard for it.Choose right books and study materials.There are other exams too like BITSAT,COMEDk and many more but the syllabus is same.Hence, preparing for JEE will work for all.Eligibility for  mains is minimum75% in boards but I waould say you should aim approx 90% in boards.Hope this helps.
Good Luck!