You can write all exams after you complete UG or if you are in last year of required UG course. So if you are doing PG you can give exams. You can't give all exams at a time so, try to choose one and focus on it.
Every exam require atleast an year of coaching.
For reputed coaching search on quora for exams and city you want. Eg. for UPSC LA excellence in Hyderabad is famous.

Anugraha B Ed College, Bangalore

Admissions Courses & Fees

Shortlist

Brochure

Joint Entrance Examination (Main)

Answer Key | Eligibility | Application | Preparation Tips | Question Paper | Admit Card | Result | Accepting Colleges

Get Updates Brochure

Graduate Aptitude Test in Engineering

Eligibility | Application | Exam Pattern | Admit Card | Preparation Tips | Books | Question Paper | Accepting Colleges

Get Updates Brochure

Common Admission Test

Eligibility | Application | Exam Pattern | Admit Card | Preparation Tips | Result | Answer Key | Accepting Colleges

Get Updates Brochure

UPSC Civil Services Exam

Result | Eligibility | Application | Exam Pattern | Admit Card | Selection Process | Answer Key

Get Updates Brochure

Tamil Nadu Common Entrance Test

Result | Application | Admit Card | Preparation Tips | Answer Key | Counselling | Dates | Accepting Colleges

Get Updates Brochure