IIM is a courses which is done after under graduation or it is done in pg I am not understanding it?

First of all IIM is an institute not a course. In IIMs course that is done is MBA in different specialization. And it is done after graduation which is also called as Post Graduation course (PG) both are the same thing.

For getting into IIMs you need to sit in CAT exam and get a very high percentile in the exam to get a call for next round of interview and WSAT. You should also know that CAT marks are not the only criteria for getting a call. After every CAT exam every IIM makes a merit list based on your aggregate score which comprises of many factors such as CAT marks, 10th 12th and graduation percentage, work experience, engineer or non engineer, gender with different weightages for different IIMs.

After second round again a new merit list is made including interview and WSAT which along with CAT exam marks have the highest weightage in final list which is used to give admission.

So pay attention to your profile along with your CAT exam after graduation to get admission in IIMs.

All the best!!