Hello Subhashis Paul.
First of all I like to say its really nice go hear that you are willing to join defence services. To clear the entrance exam you need to have your basics strong in English current affairs and maths.
These 3 are the game changer for NDA entrance exam. And to improve your basic the best way is practice. I suggest to buy arihant books they have previous year questions/papers and sample paper. It cost around Rs 500-600 you can easily purchase it from market.
All the best for your exams.

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