IIM

To get a call from a good IIM college, you need to score more than 600 marks, and yet to come in top 100 in the ranking to get a best college of IIM.

You have completed your B.Sc, so you are eligible for applying for MBA now, for apply for MBA, you need to give entrance exam subjects CAT CMAT XAT NMAT CET, after getting your score, you need to apply for the colleges on the basis of your marks. After this process, you are going to give some session like GDPI, after all you are going to get call of your admission.

Business is better, as you can use all of your abilities like Research, Innovation, Product Development, Promotions etc.

You can do anything you want to. You are a science student, you can take either B.Sc or B.Tech for graduation. After that, you have to fight an entrance exam to get admission in IIM that is CAT. After clearing this, you can get admission here.
Hope this helps you.

Well, that's not true. That depends on the company which is recruiting you and the post available with them.

Getting at least 95+ percentile should fetch you call from some IIMS (new ones). To convert the final call, scoring 98+ would help you a long way.

CAT (Common Admission Test) is the most accepted nationwide entrance test for MBA aspirants. Yes, it's accepted by Iims but every top B school has it's own entrance test. XAT (XLRI Jamshedpur), SNAP (Symbiosis), TISSNET TISS MHCET (JBIMS), NMAT (NMIMS) to name a few. Please note that getting through these competitive exams only allows you to appear for the next round in the selection process such as GD/PI and is not the final selection in itself.

Please check for your eligibility to pursue M.Com from the criteria mentioned below:
Master of Commerce M.Com Eligibility:
For admission to M.Com. Programme, students must fulfil the below-mentioned eligibility criteria. Bachelors degree in Commerce
Minimum 50% marks at the level of graduation
Certain universities also admit students with a B.A. Economics (Hons. ). Degree
Students who have passed B. S. B.A. , B. F. I. A. and B.E. With at least 60% marks can also apply
Regards.

No, it is not mandatory. You can do MBA if you are a graduate in any field.