Exam

No, these marks and the specifically the rank you have can be used once in counseling of 2019 admission.

Some of the best IAS coaching in Tamil Nadu are:
All India Civil Services Coaching Centre
Coaching Center Chennai, Tamil Nadu
Vetrii IAS Study Circle
Coaching Center Chennai, Tamil Nadu
Shankar IAS Academy
Coaching Center Chennai, Tamil Nadu.

AIR is the abbreviation for ALL INDIA RANK. This indicates the rank of a candidate in the exam in all India level.

It depends on your interest. If you are interested in engineering then it is a safe option for your future other wise degree doesn't matter in UPSC.

https://www.ntsehelpline.com/scert-kerala-released-official-mat-sat-keys-for-ntse-2019-stage-1/
Visit the above given link to download the respective papers.

Go for M.A. Public Administration if you are preparing for UPSC CSE.

In UPSC prelims, you need to cover: general studies - History, geography, polity, economics and Science. For CSAT paper 2, aptitude and general mathematics.

There are many government exams to be conducted. 1st SSC CGL last date of form submission 25th November 19, Madhya Pradesh PSC state service exam form submission 20th nov to 9th dec 19. RRB NTPC etc. And UPSC CSE 2020.

UPSC CSE PRELIMS SYLLABUS
Paper 1 - General Studies:
This includes areas like; Indian National Movement, Indian Polity, Basic Economic Understanding, Geography. While these are static part of the syllabus, the dynamic part includes current affairs, General Knowledge and that does not proper definition, thus making the syllabus quite vast. Paper II - Aptitude Test:
Paper II is dynamic in nature and includes mathematics, reasoning, analytical ability, and interpersonal skills. All these areas require spot mental ability.

You can apply 9 times and the maximum age is 35 years as of now.