Hey

You can start your preparation as soon as possible.

You can give  CAT exam  in your last year of graduation or after completing the graduation.

 The basic eligibility criteria of CAT :

the management aspirants must have a Bachelor's Degree with minimum 50 percent marks in aggregate. The final year graduation students are also eligible to apply for the exam. There is a relaxation of 5 percent in graduation marks for SC, ST and PwD candidates.

Hey.

You can attempt the CAT exam either after completion of your B.Com Degree or while you are in your final year of Bcom Degree. And you also need to score a minimum aggregate of 50% marks in your Degree to be eligible to take the CAT exam. Though you can start the preparation for the same from the early years itself.

Best of luck