For becoming an IAS you have to prepare for the Civil Services examination conducted by UPSC (Union Public Service Commission) every year for various Government services like Indian Administrative Services (IAS), Indian Police Services (IPS) and Indian Forest Services (IFS) and Central Services such as Indian Audit and Accounts Service (IA&AS), Indian Foreign Service etc .
Since you are in 11th so firstly you will need any udergraduate degree to become eligible for giving this exam.Secondly you have to prepare yourself for the exam in very efficient way and with a goal oriented strategy as par the prevailing competition.You can take coaching in offline or online mode according to your suitability for exam preparation.
•Different stages of the exam:
1.Preliminary - Objective pen and paper based exam: It is comprises of 2 paper-general studies 1 and general studies 2(Civil Service Aptitude Test or CSAT).
2.Mains - Written exam with subjective questions.
3.Personality Test - Interview
•Duration of Exam:
-Preliminary - Each paper is of 2 hours
-Mains - Each paper is of 3 hours
For futher detailed information you can visit these links: https://www.google.co.in/amp/s/competition.careers360.com/articles/upsc-ias-exam-pattern/amp?espv=1
Hope this helps