Hii aspirant!

First of all if you are student of science that is your prime subjects includes

Physics

Chemistry

Maths

The above mentioned three subjects are mandatory for the NDA examinations. Rest your optional subjects are not a matter of concern.

If you are thinking about the posts and its specification then it's quite huge in dynamics and mainly depends upon individual's performance.  So you better start preparing yourself for the written examination first which is proceeded with medical checkup and interview.