Hello,

The eligibility criteria for UPSC is passed graduation in any discipline from a recognized university, there is no specific aggregate required in this case, candidates in the final year of graduation too are eligible provisionally to apply for the same, you can pursue engineering in any discipline of your choice, however some of the discipline enlisted as optional subjects are civil, electrical and mechanical, so if you have an interest in any of these areas then you may pursue the same, otherwise choose any graduation stream as per your preference, in order to commence your preparation for UPSC get acquainted with the syllabus, learn it by heart, start reading any newspaper such as The Hindu or The Indian Express on a daily basis focusing on editorial section, government schemes, news of national and international importance etc, once you know the syllabus you get to know what to read and what to skip, apart from NCERTs you can refer 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 & 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

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 your resources minimal, read one book many a times instead of many a book one time, as you have to retain so much of information so rely on only limited yet trustworthy resources and revision is the only key for remembrance.

Writing mains is a huge task, for nine paper it requires immense practice along with knowledge. So, you have to start practicing answer writing in advance, even in a little way, There are many sites which provide questions on daily basis you can avail that. Try to give a holistic viewpoint while answer writing by providing political, economical and social aspect. You can get topper's answer sheet from various sites, try to analyze and study them minutely, you'll get an ideas about what to write, how to express and present viewpoint in organized manner.

Go through previous year question paper which you can get from the official website of UPSC, additionally apply mock papers, and test series which you can get it from any of the sites exclusively dedicated to UPSC, it's highly important to evaluate your performance going through the preparation process.