Hello Sahil

Since you are in 12 th right now,  you can give CAT exam after your graduation in particular field of your interest. Some people crack cat in first attempt itself while some not.

CAT is not only about the entrance exam but GDPI and your academics are also equally important. You can join either online or offline classes and understand the syllabus. You can start at the time you want.But according to me start two years before the year you wish to give the exam to get better results

Read newspapers daily,stay updated what's going in the world. Do some extra curricular activities. Participate in competitions which are available online.This can improve your profile

To crack CAT, most important thing is to give mocks from relevant sources. The more you practice the more you will be better.

For now focus on your 12th as the percentage are utmost important.

All the best!