You should prepare yourself for the overall year that you are going to have. You will come to know about the academic nitty-gritty on your way. The syllabus is definitely vast as the IITs will help you develop yourself more as a person than a student.
You can find the syllabus for the first year on the official website. You will find the syllabus of different branches on their departmental pages. If you have any specific doubts you can reach out to the current students or alumni of the institute. There is an IIT Madras Freshman Forum.