Diploma

For getting into IIM, first you need to attend for PI/What rounds conducted by IIM's. For getting a call from reputed IIM's you must possess a good percentile in CAT exam. Good percentile could be at-least more than 90 for general category.
These play a major role in getting a call from IIM. An average scholar with at least an aggregate of 85% in X, XII and 65% in UG can get a chance to get into IIM.
One should be able to speak fluently in English, which helps to defeat the others in GD (in some cases) and it also helps in PI as well. Because one needs to understand what the interviewer is asking and able to communicate his/her views clearly. Next comes to writing, where in you need to write an essay on the topic given. For mastering in that round, you should know the correct grammar usage and good comprehension.
Moreover, for a successful conversation of an IIM call, it is depends on your abilities too like convincing skills, answering skills, etc. Also, there may be many factors such as work experience, academic achievements, etc., which do not count for minimum requirement for getting into IIM but if you do possess them, they tend to an added advantage.