Hi

UPSC civil services examination is a 3 three staged examination

*The first stage of the exam is Preliminary examination (objective )

*The second stage, Mains (written).

*If you manage to clear mains then you are to go through third stage i.e interview/personality test .

Through UPSC civil services examination a total of 24 services  are offered which are provided below :-

ALL INDIA SERVICES

Indian Administrative Services  i.e IAS
Indian Police Services i.e IPS
Indian Forest Service  i.e IFoS

GROUP A CIVIL SERVICES

Indian Revenue Services i.e IRS
Indian Foreign Service i.e IFS
Indian Audit and Accounts Service i.eIAAS
Indian Civil Accounts Service i.e ICAS
Indian Corporate Law Service i.e ICLS
Indian Defence Accounts Service i.e IDAS
Indian Defence Estates Service i.e IDES
Indian Information Service i.e IIS
Indian Ordnance Factories Service i.e IOFS
Railway Protection Force i.e RPF
Indian Trade Service i.e ITS
Indian Railway Traffic Service i.e IRTS
Indian Railway Personnel Service i.e IRPS
Indian Railway Accounts Service i.e IRAS
Indian Postal Service i.e IPoS
Indian Communication Finance Services i.e ICFS
Indian Ordnance Factories Service i.e IOFS

GROUP B CIVIL SERVICES

Armed Forces Headquarters Civil Service
DANICS
DANIPS
Pondicherry Civil Service
Pondicherry Police Service

Yes, you can start your preparation for UPSC civil services examination after 12th but you will be eligible for writing UPSC civil services examination only after your graduation or bachelor's
as to be eligible:

You  must  have  a  bachelor's  degree in any, discipline    from a  recognised university and you  need to  be at least  21  years  old .

Many aspirants start preparing for UPSC civil services examination after their class 12th along with their graduation you can do the same and prepare for it  following the below tips :-

To start your preparation  first of all go well through syllabus  , you can check the complete syllabus at
https://competition.careers360.com/articles/upsc-ias-syllabus

Top books which you can follow for UPSC examination preparation are listed below subject wise:-

HISTORY

*NCERT XI (Ancient & Medieval)
*NCERT XII (Modern Indian History)
*Facets of Indian Culture Spectrum
*From Plassey to Partition - A History of Modern India

Indian Art & Culture

*Indian Art & Culture - Nitin Singhania

Polity

*NCERT IX-XII
*Indian Polity M Laxmikanth

Geography

*NCERT VI X (Old Syllabus)
*NCERT XI, XII (New Syllabus)
*Certificate Physical Geography G C Leong
World Atlas

Economics

NCERT XI
*The Indian Economy Sanjiv Verma
*Indian Economy by Ramesh Singh
*Economic Development & Policies in India Jain & Ohri

Environment

*Class XII Biology (last 5 chapters)
*Environment by Shankar IAS Academy

International Relations

*NCERT XII (Contemporary World Politics)
*Current Affairs ( read newspaper )

To get the complete list of books go through
https://competition.careers360.com/articles/books-for-upsc-ias

*Make a routine or a time table and try to religiously follow it , take short breaks in between your studies.

*Read newspaper daily and keep yourself updated with the current affairs

*Choose your optional subject wisely , choose the subject you have interest in.

*Make short notes for each subject, it  will help you in your revisions a lot as syllabus is wide

*Revise :- there's no point of preparing for the exam if you couldn't recall anything in the UPSC Examination so you should stress on revision as well.

*Solve previous year question papers and mock test papers ;- You should solve these , it will give you idea of types of questions asked and their difficulty level and will help you a lot.

*Also work on your communication skills and personality development as it plays an important role in your interview round

For more tips go through

https://competition.careers360.com/articles/how-prepare-for-upsc-ias

If you have any other doubt then feel free to ask them :) we are always here to help you out and guide you

Thankyou