Anjali
Firstly you need you understand that to get into one of the IIM's you need to perform well, I literally mean really well, in Common Admission Test(CAT), if your percentile is high enough you will get a call from one of the IIM's(maybe more than one) according to your score.
Every IIM has there own way to select the students but more or the less it's quite similar. One has to clear Group Discussion and Written Exam to be eligible for the Personal Interview then if you are lucky enough you might get admission.
Breakup of the final marks is like :
60% of the CAT score
20% for your past academic record
rest on your personal interview
These stats may vary from college to college and from year to year, but more or less it be same.