After doing aircraft maintenance engineering from JRN IAT from Delhi. Will I be eligible for CDS exam?

Hi, the basic educational eligibility for both of these exams is a Graduate degree from a recognised university. Without Graduation degree, you cannot appear in either SSC CGL or CDS.

For preparation of both these exams please refer:

SSC CGL Preparation - https://www.shiksha.com/exams/ssc-cgl-exam-pre

It depends upon student to student whether they want to join coaching or not. But going for coaching basically provide a structured approach, expert guidance and also focus exam specifically that will also help in improving the success rate. Candidate will get the updated study material in the coach

Doon Defence Dreamers as per the results and reputation is considered to be the top choice institute for the CDS preparation. But before joining any institute, it will be better to visit the campus in Dehradun, evaluate the faculty, infrastructure and the guidance provided there.

There are several CDS coaching institutes in Dehradun which provides comprehensive written and SSB guidance. Some of the academies are mentioned below:
Centurion Defence Academy
Prasad's Banking SSC and CDS Coaching Institution
Major Kalshi Classes
Dehradun Defence School, etc.

There are various coaching institutes for CDS in Dehradun. Some of them are Doon Defence Dreamers, Centurion Defence Academy, Brigadier Defence Academy, etc. It will be better to visit personally to the coaching institutes in Dehradun, take the reviews regarding faculties, infrastructure, requiremen