CBSE 12th

Following the NCERT book is the best option, if you are from CBSE. Other than those, you may follow S. L. Arora or H.C. Verma for Physics, ABC for Chemistry and R. S. Aggarwal or R.D. Sharma for Maths.

Yes, you can apply. You have to provide proof of passing qualifying exam at the time of admission.

Don't ask this question to others, it is your decision.
What do you want to do in your life?
There is many options. And, every Field have their own value. But, do something that you really want to do. Something you like badly. Something you want to do. You can get suggestions from parents and teachers. But, it is your choice.

The syllabus of class 11th and class 12th carry a lot of weightage in BITSAT. It is a concept based paper, if your concepts of class 11th and class 12th are clear, and if you have a good command on mental ability and logical reasoning, you can easily clear BITSAT.

If you want to score 90+ in all subjects, then you need to practice and study on a daily basis. Physics, Chemistry and Mathematics should be practiced and studied daily by devoting equal amount of time to all the three subjects. You should study CS on alternate days and you should never neglect English as it carries the same weightage as other subjects carry. You can study English at least one day a week.

There is nothing impossible until you have the will to do it. Everything is easy until you say it's easy the more you say it's difficult the more you find it disturbing. There are people who have taken Science in class 12th and jumped to commerce in degree and are happy about their course. It all about your dedication towards your course and your interest to learn new skills every day and be a knowledge explorer. I say if your heart and mind says yes, you can definitely do it. Be confident of what you are choosing and think about sustainability in the same. All the very best for your future.

I suggest you to go through this link to know about all the details related to the exams, eligibility, date and other information. These link has details suggested by Shiksha.Com. I have shared the same for your reference. https://www.shiksha.com/science/exams/amp/iiser-entrance-exam.

Dear Mridusmita!
With this profile, absolutely you can get the admission in Guwahati University for MBA Program .
Good Luck.

For BCA, you must have any one of subject must be mathematics. You may be from commerce, arts or Science but must have one subject as mathematics. Otherwise you will not be eligible for BCA.