The written examination of NDA is devided into two parts:

Mathematics

General Ability test

Mathematics: This section is consist of 10+2 syllabus so if you are in class 12th then study well as it will help you for NDA exam.

Try to give your self a time limit and solve the problems within the stipulated time.

General ability: Understand the basic concept of Biology, Chemistry and try to solve the problem. You need to also work on Biology, history, Civics & Economics.

Here are some preparation tips you can prefer :

Check complete syllabus so that you an take an idea about the important topics.
Strong your basics – Clear the concepts and formulas of your Class VI, VII, and VIII.
Read Newspaper, Magazines, Gk books etc. enhance your overall general knowledge.
Brush up your Grammar skills. to improve your english prefer class 10th & 12th lvel English grammer book.

Make a schedule for every topic here is the complete guide on proper time management tips for NDA Exam.

Improve your skills with NDA Online Practice paper and Mock test.
work on your weak areas.

Good Luck!