My 10 CGPA is 8.4 and the 12th mark is 86%. Should I prepare for the IIM Ahmedabad?

Yes, IIMs particularly IIM Ahmedabad demand good academic records. But, IIMs require more than academic performance. IIM admissions usually include:

• Common Admission Test (CAT), is a nationwide management admission test, that must be taken. CAT scores will influence IIM admissions.
• Academic Record: Your academic record matters in the selection process. Your bachelor's degree performance will be assessed alongside your 10th and 12th grades. 
• Job Experience: Although not required, many IIMs prefer candidates with job experience. 
• Personal Interview: IIM interviews shortlisted individuals to evaluate their communication skills, leadership potential, and other non-academic characteristics.

Thus, while your academic record is a plus, you must do well in the CAT and other selection processes to get into IIM Ahmedabad. It's also worth mentioning that thousands of applications compete for a few IIM places. It's worth preparing for the IIMs with the scores you have, just focus on the remaining parameters which are in your hand now.