You find out they are hiring online at armstronggarden.com. You must pass a test that takes about an hour at the store - general plant knowledge you will definitely need on the job. All they test you on, customers will ask - probably starting with your first day.
My advice if you want to work there - make sure you know your plants and pests. Also, fertilizer knowledge and customer service, because they test you on that, also. If you are not initially hired as full time, they will use you during the busy season, then cut your hours to nothing.