As a semi-decent gardener (I can make things grow, I just hate weeding), if you are going to be expected to give customers gardening advice, these are the things I'd advise you bone up on first:
1. What grows well in your area - which means knowing if your area is prone to frost, drought, salt (i.e. near the coast with sea breezes), soil type, average rainfall. Your company training should cover some of it, but it's worth your while doing a bit more digging (boom! boom!) to get some ideas about what are 'easy' plants for your area and what are trickier.
2. Make sure you understand which fertilizers are needed for different soil types. This is the one that drives me crazy in the chain hardware stores with garden centres attached, as opposed to nurseries - staff who don't understand the difference between a wetting agent, a soil conditioner, a soil lifter, a plant food, a compost and a fertilizer. They all do different things, and different ones are needed for different soil types and different outcomes; they aren't interchangeable. I **hate** seeing people who aren't sure what they need being told to buy a nitrogen-fertilizer when they've got heavy clay soils and need a soil lifter, or people with sandy soils being sold wetting agents when they need compost.
3. Get to know your garden tools. Seriously, make sure you know the difference between a spade and a shovel (and between shears and secateurs). The quickest way to piss a serious gardener off is to be dumb about tools, ESPECIALLY when it comes to the quality of the tool. I'm happy to spend $20 on a cheap rake because I'll only use it to rake over topsoil once a year, but a garden fork I expect to last 20+ years and I'm prepared to pay for it; if you're going to sell me tools, I want to be sure you know what you're talking about.
If you can, get to the library and read a few gardening magazines from the last two years. Like everything, gardening has its trends, and for the type of place you're working, you're going to get more of the trend-followers and fewer of the hard-core gardeners (who are unlikely to ask you many questions beyond 'do you stock...' and 'where do I find the...')
Good luck with it
