a. An MCQ test

b. A Subjective Test

c. An Interview

For Entrance Test, be through in all the maths taught in school at the +2 level. You can find syllabus on ISI Admission site. The exam follows the pattern of most high school mathematics competitions (RMO, INMO etc).

Books that you should follow:

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

The first book may seem tough at first but stick to it. Sit and think about the problems, it will help your mathematical abilities a lot, although the level of problems is mostly beyond what's asked at the entrance exam, it will help nurture in you a right way to approach problems. Don’t disappoint, if you are unable to solve some problems, it happens to all, just try problems again and again, that is important.

The third book discusses many concepts that are not taught in school (Pigeon Hole Theorem, Linear Congruence, Fermat's Little Theorem, Wilson's Theorem, some useful theorems in Geometry),

The second book contains past year question papers.

You should complete these three books. Do previous year papers of ISI Entrance, as well as previous year papers of RMO and INMO.

Regarding Interview, you should clear your concept, if you are saying something in the interview, then you should able to explain every word of your sentence. But if you can clear the entrance test, then cracking interview will not be a difficult job.

Since you have one year, that it is enough time for preparation. But you have to decide now, that are you really serious about B.Stat or B.Math or B.Sc. in Statistics or B.Sc in Mathematics. If yes, then you have to focus more on Mathematics. Entrance Exam of ISI is not easy and also there are only a few seats for B.Stat. You have to prepare well.

Hope your query is solved!

All the best!