Government & Defence Entrance Exam

Candidates must have taken the Combined Defence Services (CDS) admission test; the candidates must be an Indian citizen, not more than the age of 24 years and should have a bachelor's degree from any University recognised by the Indian government. The application normally occurs between October and November while the examination takes place in February and September. The application has to be made online through the Union Public Service Commission's website for which general category candidates are required to pay INR 200 as fee and there is no fee for SC/ST/OBC candidates. The exam consists of 3 papers: The English Examination is 60 marks, General Knowledge is 100 marks and Elementary Mathematics will be 140 marks in total.

NDA preparation is a multidimensional approach imbibing academic knowledge with physical fitness and mental prepare. Here is a blueprint to get you started: 

Know the Exam Pattern and Syllabus: Well the NDA exam is divided into two papers:
Mathematics (300 marks)
General Ability Test (GAT) (600 marks), which includes:
English
General Knowledge (Physics, Chemistry, General Science, History, Geography, Current Affairs)
So, the first thing that you need to work upon is a clear and thorough understanding of the detailed syllabus for both these sections. You can refer to the official notifications by UPSC or trusted educational resources like Byju's or Gradeupudy Plan*:

Daily timetable with allocated time for every subject.
Revision and solving the previous years' question papers to get acquainted with question patterns and time management.
Focus on the weak areas, spend more time on preparation for such areas.
Mathematics Preparation:

Mathematics topics include Algebra, Trigonometry, calculus, and co-ordinate Geometry. These are very essential topics in terms of scoring marks.
Practice problem sets given in books like R.D. Sharma and M.L. Khanna.
Practicing continuously on online tests and solving problems of last year's papers build up the speed and helps gain accuracy really well .
General Ability Test (GAT):

English: Develop comprehension, vocabulary, and grammar. Grammar books by Wren & Martin and vocabulary books by Norman Lewis will help.
General Knowledge: Read newspapers, magazines, and online news sites. For static GK, read Lucent's GK or NCERT books on History, Geography, and General Science.
Physics and Chemistry: NCERT books for Classes 9 to 12 serve as a good base. For Physics also look at H.C. Verma.