how to prepare for NDA India examinations

First you need to understand what kind of paper pattern you will see:
From little search on internet I was able to find following :

Paper Patern
The examination consists of 2 Papers, Mathematics (300 marks) and General Ability Test (600 marks) with objective type questions. The duration for each Paper is 2 ½ hours. Negative marking is applicable.
Syllabus
Paper I- Mathematics: Algebra, Matrices and Determinants, Trigonometry, Analytical Geometry of two and three dimensions, Differential Calculus, Integral Calculus and Differential Equations, Vector Algebra, Statistics and Probability
Paper II - General Ability Test
Part A: English
Part B: Physics, Chemistry, General Science, Indian History, Freedom Movement in India, Civics, Geography, Current Events

Further you need to read this :

Course is extensive so study everyday 2 − 3 hours on regular basis. Course may be divided in small chapters & proper study plan will bring sure success.
Previous years question papers can be used to know the exam pattern & the important topics to shorten the course.
For the Social Science (History, Geography, etc) part, class 6, 7, 8 must be given just a rough reading; class 9th & 10th books will cover all the aspects with additional information to those of class 6th, 7th & 8th.
Reading monthly magazine of 2 − 3 months prior to the date of exam will be helpful.
Mathematics questions take time to be solved, therefore, time management must be kept in mind because its general scenario that candidates are unable to finish the questions those are so easy. So, while writing the exam, be aware of these easy questions, so that you can solve more questions.
All India Merit will include the marks of your written exam also, so studying hard & getting good marks is the only way for you to get good merit rank.
There is negative marking, avoid giving wrong responses, only tick the option when you are sure. I will say if you avoid guesses, it will help you.
Keep pace with the time, avoid stress, keep calm. In the last five minutes of the exam, tick all your responses in one go in the OMR Sheet. Be conscious for leaving the space if you have not solved that question.
Science (Physics & Chemistry) will be easy for those having these subjects, but just a revision of their NCERT will do it all.
English can fetch you really good marks. So its study along with other subjects is required. Thorough understanding of PARTS OF SPEECH will help you do well in error finding.

So, essentially knowing your syllabus and being on routine can help you, if you were good in academics in your highschool and intermediate then should be even easier.