What to pursue after completing 13 years of defence service

You have two alternatives. One is to go for post-graduation in the subject. If there is a facility you can do this by joining in the night college or you can do by distance mode also. Once you complete your PG you can become a lecturer in any engineering college after coming out of your defence service. Many defence people are opting for lecturer posts after retirement. Some colleges will prefer you even as a principal as you have 15 years experience. You can join there and do your doctorate while working as a lecturer. You have good scope for getting a lecturer post in electrical engineering as there are many engineering colleges and almost all the colleges will have this branch,
The second option is to go for B.Ed. You can do this by distance mode from IGNOU or any other reputed universities who will offer the course in distance mode. The course is for two years. Once you are qualified in this examination you will become eligible for a teacher post in any government school or private school. But for government school, you may have to write the entrance test and get qualified in that.

With your engineering degree you have much scope to try for teaching jobs in private colleges or schools but if you want to get a job in the Govt sector teaching then you might have to go for Post Graduation or B.Ed. through a distance learning mode. Another thing is there are many Govt jobs where a small quota is reserved for the ex-army persons also so you have to look for that segment also after your retirement. There is one possibility also and that is the teaching in coaching or tuition centres. These centres have come up in small towns also and are seen in every nook and corner of it. So getting a job in such places will not be so difficult.

In fact for ex-army persons there are also various scopes in the area of HR, Administration, Security Officers, Supervisors, Clerks, Warden etc which are all very good positions in different organisations and one can choose from them as per ones interest and aptitude.

If teaching is your passion, you are not limited to classroom teaching these days. Decide what subject you would like to teach and prepare yourself accordingly. Being a subject matter expert you can even prepare a course material that covers all the disciplinary skills you learned during your defence service that made a difference in your life. If your defence experience is technical then it would be a good option to pursue teaching in engineering discipline. But, to work full time in an engineering college or an institute requires M.E or M.phil based on the norms of the governing body.

To start with, you can participate in communities like toastmaster to build your connections and network with like minded people.

Starting a YouTube channel will give an opportunity to work independently based on your time availability. You can make a decent earning in youtube depending on the quality of teaching content and total number of active subscribers on your channel.

Creating and uploading good video courses by registering as a tutor in education websites like Udemy and Udacity will give you a chance to take your knowledge to global audiences.

All the best for your post retirement life.

It would be best in your interest to opt for teaching once you retire from the defence service but you would be required to enhance your qualification at least a ME degree from a recognised university. The best recourse would be to join some private college to work as a capacity of lecturer in your stream ie Electrical - engineering and simulteneously prepare you ME in some colleges having provision of Evening - classes. If you feel some sort of inconvenience with such a persuasion, you may do the same with the distance - mode preferably through IGNOU. This university is pioneer in terms of imparting quality education in the different spheres. Once you complete the same with an aggregate of 55 percent marks, you become eligible to appear for NET test conducted by CBSE. The same test consists of two papers first one include current affairs, Reasoning and some Numerical - mathematics and the second paper would deal with your stream with which you have passed your Engineering.
Upon passing the said test, you will have the chance of joining as a lecturer in a constituent college. Later, you can go ahead for the doctorate degree for better career prospects in the teaching line.
If interested for being a school teacher, you will have to persue B.Ed either through the regular mode or through the distance one from a recognised institution. Even though B.Ed would take time in completion, the end result by joining a school as a teacher may not stimulate you much. The first option seems to be prudent.