now i am in 12th and should write 12th exam in 2021. Then when should I write NDA exam.
You can appear in NDA-2 2020.A candidate's age must be in between 16.5 years to 19.5 years.If you appear in NDA-2 2020 your results must be declared by June 2021 if you qualify.In case if you couldn't qualify you can still appear in NDA-1 2021.You must check the eligibility criteria before applying.
National Defense Academy Examination is held twice a year, in September and April by UPSC. There is no limit to attempts so a student can appear for several times until the age limit doe not exceed.
Therefore depending on your age limit you should see when you are prepared for the exam. It includes English, General Studies and Mathematics (11-12th). Accordingly you can appear for the exam in September'20, April'21 and September'21.
If you are prepared with parts that can score qualifying marks then attempt for NDA 2, September even.
And for NDA (1) and NDA(2) in 2021, your syllabus would be over by that time so you can easily attempt the exam.
Please mind the age limit before applying for the exam, which is 19.5 years maximum.
All the best