To join National Defence Academy a candidate must appear in the entrance exam conducted by Union Public Service Commission.

You can check below eligibility criteria

- The upper limit is 19.5 years for appearing in NDA

To know in detail you can refer below link

NDA

You can follow below books for your preparation

-Manorama Yearbook

-Mathematics for NDA by RS Aggarwal

-Objective General Knowlegde & Current Affairs by Disha Experts

-Quantitative aptitude for competitive examinations by R.S. Aggarwal

You should also refer to previous year papers to understand the exam pattern and level of the questions for your practice.

Good Luck!!