Hello There , 

Usually the students fill up the form for CAT in their final year of college . As you are pursuing B.com , you can apply when you are in 5th or 6th sem . But you have to be sure that you clear your backs first.

regards

Tanushree Seth

Hi Cherry,

Final year graduation students can apply for CAT. So, you can ideally apply in 5th or 6th semester of your graduation. Make sure that you clear the backlog by the time the admission process of IIMs and other CAT participating institute starts. Check complete CAT eligibility criteria and other information related to CAT 2018 through the links given.

Hope that's helpful. All the best.