Hello,

Firstly you need to get acquainted with the syllabus of UPSC, and learn it by heart, you can get it from the official website of UPSC, when it comes to starting your preparation then NCERTs are highly recommended, and inculcate the habit of newspaper reading from the beginning,

For Prelims, you can go through the following books:

• History; NCERT Class XI and XII/India's Struggle for Independence by Bipin Chandra/For Indian Culture, refer art and culture by Nitin Singhania
• Geography ; NCERT Class VI to XII, GC Leong's Physical Geography, World Atlas
• Economics ; NCERT Class XI,Indian Economy by Ramesh Singh
• International Relations ; NCERT XII Political Science with current affairs
• Polity ; Indian Polity by Laxmikanth
• CSAT ; Verbal and Non-Verbal Reasoning by RS Agarwal, M Tyra's Quicker Maths

When it comes to  Mains Exam, you can refer the following books for four General Studies Paper;

GS I - For history, Class XI NCERT book with India's struggle for Independence by Bipin Chandra. India after Independence by Bipan Chandra/ History of  Medieval India by Satish Chandra, For Geography refer World Geography and Geography of India by Majid Hussain, GC Leong's Physical and Human Geography with NCERT Class XI book.

For GS II - Polity and International Relations, refer Indian Polity by Lamikanth,India's Foreign Policy by Rajiv Sikri

For GS III -  For Economics refer Indian Economy by Ramesh Singh, read ARC report along with Economic Survey and Fiscal Budget.

For GS IV - For Ethics refer a book named Lexicon by Niraj Kumar, also you can go through study material of IGNOU.

Keep resources minimal, read one book many a times instead of many a book one time.

Start practicing answer writing in advance, even in a little way, writing mains is a huge task for nine paper, it requires immense practice along with knowledge. There are many sites which provide questions on daily basis you can find that very useful.

Refer previous year question paper from the official website of UPSC,  and practice it, in addition apply mock papers, and test series which you can get it from any of the sites extensively dedicated to UPSC to evaluate your preparation level.

Hello,

UPSC has a vast syllabus and you will find plethora of books just for single subject so it becomes tough to choose one especially when you are in initial phase of preparation. Different coaching institutes and toppers prefer different study material. Here I have tried to provide you brief knowledge about books and study material that may help you in your preparations:

1. Start from NCERTs. All aspirants know the value of NCERT. Try to read these books multiple times. Basic NCERTs form Class 6-12th would really help you.
2. Read some basic books for UPSC like Spectrum for Modern Indian History, Laxmikanth for polity, GC leong for some part of geography, etc.
3. For economics try to get a brief knowledge about Economic survey.
4. For current affair you can go for monthly compilations provided by some coaching institutes like Vison IAS.
5. You can not skip newspaper. It is also advised that with changing UPSC pattern you must read two newspapers , although its tough but you can try.

If you will follow these study materials, you will be able to cover 80% of the syllabus.

ALL THE BEST