One thing I forgot to mention is safety. Safety always comes first!
If you can't reach something, get a ladder (these are only for reaching things in the back, not on the shop floor) or kick stool. If you're facing the top shelf, there will come a point where you have to get down, move and go back up - don't over-reach as you can fall off. If you are using a kick stool don't leave it on the shop floor unattended - customers don't look where they're going and might trip over.
If you're opening boxes you may get given a safety cutter. These are generally safe, but again don't leave it on the shop floor unattended.
Don't carry too much. Large boxes of wine etc are heavy, but are also breakable if dropped. Tins (eg beans) come in large cases which again can be heavy. You shouldn't be expected to do any lifting on your trial as you won't get training, but avoid doing anything dangerous. Don't feel like you have to show off with how much you can lift.
If you see any spillage, don't leave it alone. It's tempting to run off and get cleaning equipment, but someone could slip on it whilst you're gone. Stand over it and warn customers. Try to attract a colleague's attention (which should be easy as you're unlikely to be left alone) and then one of you can guard whilst the other fetches. If a customer interrupts then don't wonder off; it's tempting to go to show them something as you think that's good customer service, but your main priority is the spillage. Things like cream and washing up liquid are sticky and difficult to clean, so don't rush as you might miss a bit.
If you find anything dumped on a random shelf, put it back. However, if it's supposed to be chilled or frozen it can't be sold as it has got warm whilst sitting on the shelf. It needs to be recorded, so don't just throw it away, but there will be a special area to deal with damaged and unsellable products.