Hi

It's completely your call but yes preparing for jee main will definitely will help in your NDA examination, as it has maths with same syllabus of 11th &12th. Moreover, if you prepare for jee main then you would find NDA paper comparatively easier because it's difficulty level is lower than jee main but yes you will have to additionally study English geography etc for GAT paper of NDA.
So, take your time and then decide. If you decide to prepare for JEE main then you can join Akash or any other good institute but if you are looking for an online platform for JEE main preparation then careers360 offers you jee main knockout which has AI based program for jee main preparation with video lectures, doubt clearance facility etc:-
https://learn.careers360.com/knockout-jee-main /
If you don't want to prepare for jee main then can prepare for nda following the below steps:-

--------------------------) First of all know the syllabus and examination pattern.

Syllabus:- https://competition.careers360.com/articles/nda-syllabus

Examination pattern:-

NDA Examination pattern  for Maths

NDA examination pattern for General Ability Test

For detailed examination pattern checkout:-

https://competition.careers360.com/articles/nda-exam-pattern

---------------------) The top books which you can follow for the preparation of your NDA  examination  are provided below:-

>>>>>> MATHS

>>>>>> GAT

For more about books for nda visit :- https://competition.careers360.com/articles/best-books-prepare-for-nda

------------------------) Practice questions:- after finishing each chapter you should practice mcqs in case of both Maths & GAT

------------------------) Short breaks :- take short breaks in between your studies, studying continuously can be brain tiring and it becomes difficult to focus and study with concentration, so take breaks to relax and refresh your brain.

------------------------) Make short notes:- You should make chapterwise short notes .These short notes will be a great revision tool for your examination.

------------------------) Set  goals :- set daily & weekly goals to cover your syllabus and meet them without any fail.

------------------------) Follow a routine or timetable :- you can make a routine and religiously follow it and try to give at least 7- 8 hours to study each day in your routine.

------------------------) Revise:- Revision is very important otherwise as you move forward in your syllabus, you start forgetting what  you have learned before and you won't be able to perform well in your examination.

------------------------) Solve previous year NDA question papers and attempt mock test :- Solving previous year and mock NDA questions papers will let you know the examination pattern, your preparation level etc and  it will be of great help to you . For nda previous year papers checkout our page at:- at https://competition.careers360.com/articles/nda-question-papers/amp

For more tips checkout:- https://competition.careers360.com/articles/how-to-prepare-for-nda

Thank you