You need:
-business. Understand accounting, retailing, marketing. This can be done at most any college.
-some horticulture (but you don't need to know the more scientific aspects of how plants grow). Steer clear of botany, IMHO.
-college flower arrangement program. I have no idea which colleges do this, but this is a pretty important skill.
If I had to suggest a particular school, it'd be Niagara College. I'm sure you have plenty of other good options.
My advice beyond that is to work in a flower shop during/after you get your training. This will help you to familiarize yourself with the different varieties, and get a better sense of what kind of business you'd like to start.