UPSC Civil Service Exam consists of three level-
Preliminary numbers are not added in the final list, you just need to score the required cut offs against your category.It mainly consists of GS Paper-I 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.
and CSAT includes topics such as basic numeracy, comprehension, logical reasoning and analytical ability, general mental ability, decision making and problem solving, interpersonal skills including communication skills. Both consists of 200 marks, MCQs based. Prelims is mcq based, but still it requires knowledge coupled with speed.
Mains is more of a subjective type, one needs to write answer for Four General Studies Paper(GS-I includes Indian Heritage and Culture,History and Geography of the World and Society, GS-II includes Governance, Constitution, Polity, Social Justice and International Relations, GS-III includes Technology, Economic Development, Bio-Diversity, Environment, Security and Disaster Management, GS-IV includes Ethics, Integrity, and Aptitude),Two Optionals, One Essay, two language paper, there are total nine papers here. 250 marks for each paper, total is 1750 marks followed by Personality Test of 275 marks.
Now, coming to your question preparation for UPSC anyway takes two years, that's one year of preparing for the exam and another year of appearing for Prelims, Mains and Interview.
You can go through the gradual steps as mentioned below in order to start your preparation for UPSC:
Be well acquainted with UPSC Syllabus, learn it by heart, that's the most crucial step to crack this exam, to know what to read and what to skip. In addition to this, never ever make a differentiation between prelims and mains, what I mean is do not prepare separately for each, because nowadays what's happening is even in prelims, UPSC frame questions based on Mains syllabus, So, the first towards preparation shall be a integrated preparation for both.
Read any newspaper like The Hindu, or Indian Express on a regular basis to be updated as well as gaining knowledge which shall be highly useful to you in future exam preparation part.
Start with basics, for each subject, you can do with NCERTs for this.
As you progress, move on to other relevant books, keep your resources minimum like Polity by Laxmikanth, History of Modern India by Bipin Chandra etc.
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.
Practice previous year question paper which you can get from the site of UPSC,in addition mock papers, and test series which you can get it from any of the sites extensively dedicated to UPSC, , it's highly important to evaluate your performance while doing so.