In my experience, most nurseries will hire just about anyone with a pulse as a
salesperson. I started as a cashier at this nursery arround the corner, not
knowing what I wanted to do with my life. Two years later I was in charge of
the cultural department taking care of 16 acres of retail plants. I also did
sales in between, and that is a good way to learn, but like someone else said,
you can also learn bad info that way.
With schooling you will learn the right info, but then are in conflict with
what the garden center wants you to tell the customers and what you know you
should tell them. You might also consider taking the master gardener program
in your area. That will teach you the basic gardening stuff and get you
started.
Extremely basic information, but for some reason it impresses them hell out of
employers and people that dont know any better.

Toad