in which year of bba i should start preparation for CAT

You can start from the very first year only if you feel that your basic concepts are weak but one year of preparation will usually suffice. But if you start from the 1st year you'll have enough time to prepare well for the exam. But if you'll be appearing for CAT in your final year of graduation then do take some mock test series then only as it will help you in time management and will provide you the types of questions generally asked in CAT.

All the very best