Hi there. 

First things first, for becoming an IAS officer one has to write the UPSC examinations that occur in three stages: Preliminary, Mains and Personal Interview. 

The Preliminary exams is just for the sake of qualifying to Mains. Your preliminary exam's score won't be considered in the final score. The final score consists of Mains and Personal Interview. Preliminary is an MCQ based exam whereas Mains exam is a full-fledged written exam with an optional subject. 

One needs to have a graduation degree in any discipline from any recognized University to be eligible to write the UPSC examinations. 

It is advisable to start your preparation from the very first year of graduation. Start with the basic NCERT books of Class VI-XII of History, Geography, Economics, and Science. 

Slowly, build momentum and start with Indian Polity book by Lakshmikant. For a proper and full booklist head to Mrunal's blog or Unacademy's website (Roman Saini's lessons). 

Develop a habit of reading The Hindu daily to be updated with current affairs. Read the Yojana Magazine regularly. 

For more information on UPSC exams like the syllabus and as such kindly refer to the following link: 

Https://competition.careers360.com/exams/civil-services-examination

Wishing you luck!