You must follow these steps for preparing for NDA:-
Check out the syllabus for the exam.
Buy all the required books.
Study NCERT books, according to the syllabus.
Make notes, self notes are very helpful in studying for competitive exams.
Leave half page at the end of every topic and two pages at the end of each chapter.
Once done with NCERT, use help books, you can alternatively also use some online coaching material or some offline coaching material too, just stick to one thing while preparing for the first time. Complete one thing, then move to another.
Once done start solving previous year questions and question banks and mock exams.
Hope this helps. Thank you.
Here are the links:-
There are two papers in NDA first paper is maths for which you'll have to prepare as jee level. NDA maths paper is quite standard and requires speed. General awareness is a paper which is a mix of many subjects like physics, chemistry, biology, general knowledge etc so there are books in the market you can purchase to prepare for it.
Check the link
You need to clear all your concepts and make your basics strong first of all to crack the examination.
For chemistry you need Alkanes, Alkenes, Alkynes, Basic Principles and Techniques of Organic Chemistry done well because of which you can clear the further chapters Haloalkanes, Haloarenes, Nitrogen Compounds, Alcohols Phenol and Ethers, etc. For inorganic part, make your Nature of Chemical Bonding concept clear and you'll go fluently through all the blocks s,p,d and f.
For physics, you need to practice more and more mcqs and numericals for stay on the top. So make sure for every chapter you have to read the synopsis well and make a formula list and then SOLVE as much as you can.
For mathematics, it's again about more and more of solving. The more you solve, the more varieties you come across which helps you get explored ti lots of different types of questions which helps you in the examination in a better way.
Best wishes. Thank you.