You need to clear the AIIMS entrance exam. Besides it in Class 12 boards for general category  a minimum of 60% in each of the following subjects: Physics, Chemistry and Biology. This aggregate is 50% for the reserved category. 

To apply for aiims u require 60% marks aggregate in physics ,chemistry and biology for general and obc and 50 % for ST, SC

In order to be eligible for AIIMS you should have 60% aggregate marks in P/C/B and English for general Category. For SC/ST you should have 50% aggregate marks.

Good Luck!!