The Indian Institutes of Management consist of 20 public,autonomous institutes for management education and research in India.
To be honest cracking IIM is itself a big achievement as such is the aura of this institution in India as it provides excellent placement as well as well-settled career options.
First you need to have Bachelor's degree with atleast 50 per cent marks(for General category) and 45 per cent(for SC,ST and PWD) or equivalent CGPA from any recognised university.
Basic Admission Steps to the IIM are CAT,WAT,Group discussions and Personal Interview.
Applicants have to register and appear to the CAT Exam with minimum 90%Percentile. Applicant must score 80%  percentile in each of the three sections of the CAT exam.In short some parameters which looked after while shortlisting are:
CAT exam score,10th marks,12th marks,Graduation marks,work experience and Gender diversity.
Hope it helps!

Hello,
To join IIM, you have to qualify the CAT exam that is held every year by one of the IIM's in the month of november or december.
CAT 2019 notification will come by the fourth week of july and it is expected that application form will be available from 1st week of August.
As per the basic eligibility criteria of IIM, the management aspirants must have a Bachelors Degree with minimum 50 percent marks in aggregate. The final year graduation students are also eligible to apply for the exam. There is a relaxation of 5 percent in graduation marks for SC, ST and PwD candidates.

Dear Student,

All the IIMs in India usually follows two steps for admission, the candidates are shortlisted on the basis of their CAT Marks and then they are called for personal interview rounf for final selection. Also, the candidates appearing in CAT exam, must have a Bachelor’s Degree with at least 50 per cent marks from any recognised university. You can use the following link of Careers 360 to find out the list of IIMs in India followed with their cut-offs Category wise:

https://bschool.careers360.com/articles/iim-admission-criteria