Accounting & Commerce

CAT is not required for admission to BCom. CAT is an entrance exam for admission to MBA conducted by IIMs and other Business Schools.

You may look for some diplomas in finances, accounting or taxation if that interests you or actuarial sciences is also a good option.
Some of the certificate courses are also available here.
http://www.isbm.org.in/Advance_Certificate.html

Dear Rohit,
With this subjects you can think about these option:
MATHS HONS
STATS HONS
ECONOMICS HONS
ACTUARIAL SCIENCE
PSYCHOLOGY
PHYSIOLOGY
CA/CS
B.COM-FINANCIAL MANAGEMENT
BCA
So choose the programme according to your passion and interest.
All the best

Yes. CA certificate is awarded after you have cleared various exams not just one. Exams are twice every year. You can appear for both or one.
Step one is to register yourself for cpt exam. You can register online

An hons course at bachelor's level is actually a mini specialization whereas general degree will give you the idea of business administration and law both. If you wish to know the difference in what you will be studying in both, just compare the course structure of both for whichever university you wish to apply and then decide.