Ah, what the heck. Just show up 5 minutes early every day for work, on your off day you might get called in I suggest you to go in if they call you. (I went in every time they called me in, the reward? I got entire month of June off when I graduated high school to go to Jamaica, Cayman Island, Mexico, and Lake Guntersville in Alabama) Whatever they tell you to do, just do it and ask them if it the right way to do things.
It important when stocking shelves you be aware of customers, don't leave boxes, or anything in middle of aisle. That a gold mine for someone waiting to trip over it and sue the company. People will try to make you mad, just be patience and try not to let it get to you. Some of those people don't truly understand how product flows from store to store and will get mad at you for something that beyond your control. Just shrug it off.
Have fun when there are no customers.