Hi There,

Feels good to meet a student who does not wish to pursue an Engineering course, even though being in the Science stream.

But my question to you is why not choose a career in the Science stream rather than shifting to the commerce stream. Though you might have had Math in your 12th Science, but CA is more to do with accounts which does relate to Math concept partially.
In case you wish to pursue CA, then you would have to opt for a B.Com Graduation. Since you would have to dedicate your entire time towards studying CA, attending classes and practicing it back at home.

Majorly, during the time you pursue CA, please note you would need a strong internship experience which will strengthen your background and will help you to find high profile jobs. After few years of experience, you can opt for working as a consultant too.
Because once you shift to another stream, getting back to science would mean waste of a year and time and effort.

I would suggests meet CA in your locality. Get to know their work and if it still interests you, you could opt for it.
Hope this helps.
Wish you good luck...