Qualifications
Since most retail nurseries are small businesses usually run by personable people, they tend to hire staff based on personal attributes and character as much as on qualifications. This is one of those few idyllic industries in which nice people get the job.
Experience and education will, of course, win out and earn someone a better job when it becomes available, but it helps most of all to have an interest in the industry and good-natured willingness to do your part for the nursery.
There's always plenty of learning to do at work. Those high school kids who start out as inexperienced hosers can eventually learn enough about the plants they water to sell them. College-educated horticulturalists will likely always have the better jobs, though, so take that into consideration if you're looking to get into the rough-and-tumble world of plant nursery stewardship.