You can join National Defence Academy by qualifying it's entrance exam which is conducted by UPSC every year. Although you have skipped this year's last registration date for it's second paper which was on 2nd july. But you could apply next year .The exam is on 9 sep
It's eligibility criteria says the candidate must be unmarried and below 18 to apply for its papers.
You can get all the necessary details about the exam on the given website.
https://www.sarvgyan.com/articles/nda-2018

To join defence posts we recommended you to jojn nDA.

where cadets of 3 sections of the Armed Forces- Army, Navy and Air Force train together for 3 years and are well groomed for Defence roles.

Students have to appear for nDA exam post 12th and cleae SSB Interview to join such cojrses.

Entire process explained below.

https://www.apnaahangout.com/join-indian-army-officer-12th-science/