If you are clear in your goal and want a successful career in Defence then NDA is right option. NDA exam preparation needs sheer hard-work and dedication, you need to be disciplined in your studies and should follow a correct plan of action.
First of all assess your strength & weakness and identify your areas of improvement, refer good study materials to clear your concepts and fill in your learning gaps, study in group and take the help of your classmates for clearing doubts.
Build your command over subjects and revise the topics as much as possible. Above all, go through the NDA sample papers thoroughly.