Hello  Gokul,

Preparation for NDA can be a hard nut to crack if it is unplanned. So you better make a plan for studies that incorporates all major areas that can be asked in the exam. For GS, pick any regular GS book and it'll work . For English and maths if it is possible go for coachings, you will have a strong insight into not just the paper pattern but also the difficulty level .also do not overlook the importance of previous years papers and model papers. (I cleared NDA thrice, and just with the help of previous years' papers ) so do keep practicing the previous years questions. Be consistent in your efforts and work hard. 

All the best .