A majority of your required first year courses in commerce, as there is a limited number, will more than likely be prerequisites for upper year courses that you need for the CA designation. It is in second year that you will need to be taking more specific courses.
Check this website for an idea of what you're going to have to take for your 51-hour credit hour requirement:
http://www.icao.on.ca/Admissions/QualificationProcess/ScheduleofUniversityC oursesforInstituteCre dit/1014page1201.aspx
Although that's for 2008-2009, it'll give you an idea of what's been required in the past. The course list doesn't drastically change every year, so it wouldn't be too different.
And if you refer to that list and check the link that jhan523 provided, you will see the prerequisites necessary. It's always a good idea to be planning what courses you're going to take 2 or 3 years in advance (essentially, plan out all four years now so you know exactly what path you're taking as opposed to randomly choosing courses as you see fit).
As usual, consult with an academic advisor in your faculty to get the most up-to-date information.