NDA

Since age limits prevent you from being eligible for the NDA, you have a few other options for pursuing a career in defence

After graduating, you can take the Combined Defence Services (CDS) exam, which grants you officer status in the Army, Navy and Air Force.
Technical Entry Scheme (TES): Based on Class 12 grades (for students in the Science stream), the Indian Army provides direct entry for those interested in technical professions.
After graduating, you can apply for a variety of officer positions in the Indian Air Force by taking the AFCAT (Air Force Common Admission Test).
SSC (Short Service Commission): After graduation take into consideration the SSC for officer positions in different branches of the military.

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.

Few important points related to NDA vacancies are given below:

• NDA vacancies are provisional and may change depending on the availability of training capacity of the National Defence Academy and Indian Naval Academy
• Candidates are required to specify clearly in the online NDA application form, the services for which they wish to be considered in the order of their preferences
• Candidates are also advised to indicate as many preferences as they wish for NDA recruitment so that while offering appointments due consideration can be given to the preferences as per their ranks in the merit list
• Candidates should note that they will be considered under NDA recruitment 2022 for appointment to those services only for which they have expressed their preferences
• UPSC will not entertain any request for addition/alteration in the preferences indicated in the NDA Recruitment application
• Admission will be offered to candidates on the basis of NDA results of the written exam and SSB Interview