Hi Aspirant,
IIM stands for Indian Institute of Management.
The Indian Institutes of Management (IIMs) are a group of 13 public, autonomous institutes of management education and research in India. They primarily offer postgraduate, doctoral and executive education programmes.

Each IIM is autonomous and exercises independent control over its day-to-day operations.
Along with the IITs (Indian Institute of Technology), AIIMS (All India Institute of Medical Sciences) and Central Universities, these are institutes of national importance set up by the Government of India to provide high quality education that is also value for money.

The selection process for the IIMs include an online admission test (that is taken by more than 150,000 aspirants every year) followed by personal interviews.
Well,if you wish to do MBA from IIM,get yourself a bachelors degree first.
The criteria is that the applicant should have a graduate degree of must be in the final year of the degree.Also,they need to have scored 50% or more marks in the same.
So,you cannot get into an IIM after 12th.
Hope this helps.