Hello,

Firstly you have just passed your 12th exam, in order to appear for UPSC you need to pass graduation in any discipline from a recognized university, also you need to attain 21 years as on 1st August of the appearing year, now depending on the duration there are still three to four years to go for appearing in UPSC exam, what you can do is once you get into college, go through the syllabus thoroughly which you can get from the official website of UPSC, this will make you know what to read and what to skip, next make a habit of reading newspaper on a daily basis, you can choose The Hindu/The Indian Express regarding this, when it comes to books start off with NCERTs and then move towards other standard books as mentioned below-

For Prelims, refer the following books:

History ; NCERT Class XI and XII, India's Struggle for Independence by Bipin Chandra, and 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

Coming 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.

Remember to keep resources limited, go through one book many a time than many books one time, revision is the key  here, apart from this go through previous year papers to know about exam pattern, marking scheme, important topics, etc, apply test series on regular basis once you start your preparation seriously, all this shall help you tremendously.