There are three sections in the entrance exam and the level of maths is the same as what you studied in your 12th. The exam follows the pattern of most high school mathematics competitions (RMO, INMO, etc).

The three sections of the Entrance Exam consist of MCQ, A Subjective Test, and an Interview.

The books you must know are:

1. Problem Solving Strategies by Engel Arthur
2. Test of Mathematics at the 10 + 2 level, Indian Statistical Institute, Published by Affiliated East-West Press Pvt. Ltd.
3. Challenge and Thrill of Pre-College Mathematics by B. J. Venkatachala

You should complete these all and do previous year’s papers of ISI/CMI RMO, INMO and IMO.

For Interview:

• Once you crack the exam, the interview part won’t be that difficult.
• Answer the questions confidently and honestly.
• The entrance exam of ISI is not easy and also there are only a few seats for B.Stat. 
• You have to prepare well.

Do not overthink and do your preparation well. This will help you a lot. Be calm and try to think of cracking the exam and the latter part would be an easy task for you.