Hello Irshad,

You can take the subjects of your choice but make sure that includes mathematics and english as these two are most important for the CAT exam.

For IIMs if you got 90% and above in 12th it will be beneficial for you but you don't need to worry even if your 12th percentile is not so good and if your CAT score is high you can easily enter IIMs but make sure you pass 12th with 1st division.

For IIM your CAT score must be above 97% and good 12 percentile will be great add on in your preparation for CAT.

So go for any stream and work hard for your boards.

GOOD LUCK!!

You can choose any stream for your graduation, that won't affect your score in CAT. Moreover take a stream suiting your interests. MBA doesn't require you to take up a certain branch of study in graduation and the syllabus of CAT is very different from what you'll study in graduation. So focus on both your graduation subject as well as your CAT syllabus to get into a good college. You need to have at least 50 percent in graduation and more than 90 per in CAT to get into IIMs. 

Your past academic records really matter a lot for your admission in IIMs so try to score more in graduation and not only the minimum qualifying marks. 

For IIMs B, L, A, C, K and I you need to score at least 98 percentile in CAT and for the newer IIMs try to get above 90 percentile. 

All the very best