Hello Rohit,
Many students start preparing for CAT exam in their final year of Graduation and get admission in top B-schools and even IIMs too. If you too are really dedicated to get a seat in IIM then you will have to work really hard. Assuming you are in your Graduation and wish to crack the CAT exam, Common Admission Test (CAT) is national level management entrance exam and the exam is taken by many students who have the dream to clear the exam with good percentile and get admission in top colleges.
To start your preparation, you should check the detailed syllabus and pattern of the exam. Make proper schedule to manage your studies. You can even join any coaching institute for preparation.
Good Luck!