IIM Ahmedabad - Indian Institute of Management

Well, it's good that you have a clear goal right from the start. I'll keep the answer short and list out a few things you should keep in mind to get admitted in one of the good IIMs.
1. First, make sure you do well in your studies during engineering. Your overall performance during your bachelors degree has a huge weightage. So, make sure you keep that up. While colleges often have their requirements listed out as 60%, make sure that you maintain your score at least above 75%.
2. Prepare for CAT: You might already know that to get admitted to one of the IIMs, you have to clear your CAT exam. Start preparing for this exam right now. Often people take a gap year after engineering to clear the exam. While there is no harm in it, don't prefer it. Instead, start preparing for the exam right now. If you want to take a gap year, take up a job during that year. That will get you extra points.
3. Prepare for your interview: I can't stress on this enough. Work on your soft skills while you have the time. Only 30% of the people who clear their CAT exam actually make it to a college. So don't ignore the preparations for your interview.
4. Do some short term courses: Depending on the field of MBA you want to get into, you can take up some short term courses. When you go in for admission interviews, you will be required to make a resume. Now that resume should have some weight. Apart from your participation in college fests, organizations, and academics, courses can get you some serious weightage. Plus, they'll grab the interviewer's attention and give your interview a direction that you can be prepared for.