Hello,

Preliminary is the first stage in UPSC Exam, it is MCQs based with two papers,each consists of 200 marks,its numbers ain't added to final list, you just need to score the required cut offs against your category.

First is GS Paper-I which consists of the following topic such as History of India and Indian national movement, Indian and World geography, Economic and social development- sustainable development, poverty,inclusion demographics, social sector initiatives etc,Indian polity and governance-constitution,political system,Current Events nationally and Internationally, General Science,General Issues on environmental ecology,biodiversity and climate change. you just need to score the required cut offs against your category.
The second paper is CSAT which is qualifying by nature,you need to score merely 33% which is 66 marks out of 200, it includes following topics such as basic numeracy, comprehension, logical reasoning and analytical ability, general mental ability, decision making and problem solving, interpersonal skills including communication skills.

You can refer the following books to initiate your preparation for Prelims:

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

Apart from this, read newspaper on an daily basis, go through previous year papers, practice mock papers to evaluate your preparation level.