To become an IAS officer you will need to fight one of the toughest exams in the world. It is the Civil Services exam. The course of this exam covers every aspect such as history, geography, politics, general awareness, and many more topics. You will be evaluated on the basis of Prelims and Mains Examination. And if you clear this two you would be called for personal interview. The interviewers are highly qualified and experienced officer so itd not a peice of cake to clear that interview. And to become IAS officer you need a rank below 250.
You should start your preparation as soon as possible. Start which interst you and slowly go to the subjects which you fear off. Read as much as possible. And then give mock tests and try previous year papers.