National Defence Academy (NDA) written exam consists of two sections: Mathematics for 300 marks and General Ability for 600 marks. Following are some good tips which will help you in cracking the examination:

Fix your goal and start preparing early
Understand the syllabus thoroughly
Go back to your basics, even if that means referring to your Class VI, VII, VIII, books for certain subjects.
Improve your English
Refer to the past years question papers
Solve as many mock question papers and past question papers as possible.