Hello student I'm sorry to inform you but you can't get admission into IIMs directly after diploma as to get admission into IIMs you need to appear and qualify CAT entrance examination.
CAT Eligibility - candidates must have passed or appearing graduation degree from a recognized or statutory college with minimum 50 percent marks.

For more updates refer to the link the link mentioned below - https://www.google.com/amp/s/bschool.careers360.com/articles/cat-eligibility-criteria/amp
Feel free to comment if you have any doubt
Good luck