If there is a particular nursery you have in mind, find out what they sell and start reading up on those species. That will go a long way.
I worked at a wetland mitigation company before I started working for a big nursery years later. At the wetland job we created islands from the ground up, and planted and saved untold multitudes of transplanted wetland aquatics and ferns and trees. Later on in the same company I started moving trees from a site and moving them with tree spades, moving them 70 miles south and sticking them in...every job just drifted into the next. I was pullin great oney down working for myself after the wetland job got snafued by an accident of a fellow DRUNK tree spade operator. Insurance killed us. So I worked for myself but my wife was afraid of the taxman so i went into this nursery to drive a delivery truck for them with the intent of part time so I could keep at my own work. Well, the owner talks to me and after five minutes gets the idea that I know my stuff pretty well. he tells me the driving position has been filled. I get up real quick and he's like WHOAH whoah there...wait a second..
next day I was up to my arse in water collecting lilies...made a grand that week and never looked back.
The point being is each job just sort of morphed into something better. Get your foot in the door, but apply yourself beforehand....it'll be recognized.
out.