Hey.

You can join NDA by taking their entrance test. Admissions are done on the basis of your performance in that entrance. To be eligible for NDA- army wing you need to pass Class 12th in any stream. To be eligible for NDA- Naval and Air force wing you need to pass clasa 12th with physics and Maths subjects.

The age limit to join NDA is a minimum of 16.5 years and a maximum of 19.5 years. Other than this you also need to fulfil their prescribed medical and physical fitness criteria. Application process for 2019 ended on 4th February. So you can apply next year if you're still eligible.