In order to join IMA (Indian Military Academy) you need  to appear for CDS exam.

CDS Means Combined Defence Service
This CDS Exam is Conducting by The Union Public Service Commission Two Times in a Every year

Below is the eligibility criteria for CDS

1- As per the eligibility criteria of CDS 2017 applicant must be minimum of 20 and maximum of 24 years of age and must have degree from a recognised University or equivalent.

2- Candidates, who have yet to pass their degree examination, will be eligible only if they are studying in the final year of degree examination.

To check the full eligibility criteria you can refer below link

CDS Eligibility Criteria

Good Luck!!