It is not important to study B.com for joining CA course. The eligibility for CA course is 10+2.One can register for the Common Proficiency Test (CPT) of the CA programme after passing the Senior Secondary Examination. If you pass  B.com with 50% marks you are exempted from the Common Proficiency Test (CPT) and can register for the Integrated Professional Competence Course (IPCC) .For details of CAclick here