Hello,

When it comes to preparing for UPSC, firstly be acquainted with the syllabus, take a print out and go through it on a daily basis, this will enable you what to read and what to skip, secondly make a habit of reading newspaper on a daily basis keeping in mind what all are included in syllabus with emphasis on editorials, you can make notes whether handwritten or digitally through app, when it comes to study materials, always start off with NCERT books, and then move towards  other standard books as mentioned below-

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 & Polity ;

• NCERT XII Political Science with current affairs
• Indian Polity by Laxmikanth

CSAT ;

• Verbal and Non-Verbal Reasoning by RS Agarwal
• M Tyra's Quicker Maths

You can refer the following books for four General Studies Paper of Mains Exam:

• 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. In case of 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.

Since UPSC has such an extensive syllabus, always try to keep the resources limited, read one book many a times instead of many books one time, revision is the key here, go through previous year paper to understand the exam pattern, important topics, and other such essential details related to preparation of UPSC.