Hi Yash,

For IIMs you need a bachelor's degree and have to go for CAT exam but as you want to go for IIM after 12th, this can be done now.
For this you have to apply to IPMAT organised only be IIM Indore for which eligibility criteria is passing marks in class 12th.
After clearing IPMAT which depends on the Cut-offs which further change accordingly, you will go through Intense interviews.
After this you will get selected in integrated BBA + MBA Course.
Hope this information helped you.

Good Luck for the future !!!