Hello Satyajit, 

Candidates just need to paas his 12th class with physics and mathematics to be eligible for NDA exams. Candidates who are appearing for the 12th exams can also apply. There is also an age criteria and physical fitness criteria.

I am sending you the link where you can find all the details regarding NDA exams eligibility.

https://www.google.com/amp/s/competition.careers360.com/articles/nda-eligibility-criteria/amp