After completing your graduation you can start your preparation for MBA. Being a student of IIM Bangalore, I can provide some insights which might be helpful for you.
IIMB selection is done through CAT. The cut-off for top B-schools is above the 99.5 percentile, which varies over the years.
The selection process of every B-school is almost the same. After clearing the cut-off you will get an interview call but few B-schools follow different procedures too. After getting shortlisted, you will be called to give Written Assessment (WAT).
Written Assessment:
You have to give your 100% to get selected in one of the top B-Schools in India. To make sure you get selected, start preparing for CAT well in advance, improve your writing skills and interviewing skills. Read as much as you can from books, newspapers, so that you can be comfortable in facing the selection process.