Hello,
Please share your complete academic details with us. You got 70 percent in class 12. Don't be disheartened as If you score a great percentile in cat then you have your chances to go into the iims and other top business schools of India.
Getting a seat into the iim is not an easy task. You need to work harder and harder. This is as simple as that.
The simple answer to your question is score as high as you can.
I hope this is very much clear to you.
All the best !!

Dear Akshit 

IIM conduct Common Admission Test CAT exams which is a national level exam conducted once in a year in the month of November or December.This exam gives a chance to get admission in more than 5000 seats of 20IIMs and 100 of top B colleges .

Eligibilty

Completed Bachelor's degree with atleat 50% aggregate

Or completed progessional degree 

Or 

In the final year of college

No age bar 

For getting in IIMs cutoff range upto 99.5-90.