Hey there

CAT is an exam which one can take after graduation so it is good to see that you are interested in it since from now. For CAT you have plenty of time right now so in current situation you can do best in 12th because 12th percentage matters a lot for a call from iims. Also you can start preparing for aptitude and English from right now.

Check about CAT

https://www.google.com/url?sa=t&source=web&rct=j&url=https://bschool.careers360.com/exams/cat&ved=2ahUKEwjIq5ieqvvkAhUSfX0KHU3DATMQFjAAegQIAxAB&usg=AOvVaw0WGtiQe_DqSNQSEEbxSdmI

Good luck