First, make sure you have your work permit completed and approved by your school.
Second, get ready to fill out Federal, State, and maybe Local W-4 withholding forms. Name, address and social security number is needed on all of them. You are a minor living at home with the people who are going to claim you as an exemption. Fill out all forms as Single & Zero. Also be prepared to fill out other forms such as an I-9 form. Sure it's a family business, but your uncle still needs to have it on file.
Finally, always show up to work 15 minutes before you punch the time clock. Look over the employee's bulletin board for anything new, write down your upcoming work schedule, and do whatever else you may need to do prior to actually starting your work shift. ALWAYS offer to stay over if needed.
ALWAYS strive to stay busy throughout the day. If you aren't working with a customer, clean something, straighten out something, restock something. If a customer asks for a product that is temporarily out of stock, ask your supervisor when the next shipment is due, get the customers phone number, and offer to call them when it is comes in. Customer service is what separates the local store from the box stores.
Suggestion selling! Learn what products go with what the customer is purchasing. You've seen that side bar at BPS.... "Other Related Products". You may save a customer from a return trip for something they forgot, and they won't forget the helpful advice. Learn as much as you can about the products you are selling, so that customers can ask you questions. If you don't know the answer to a question, say the following: "I'm not sure, but let me find the person that does.". Good customers admire honesty!
Remember that customers have bad days. You are working in a hardware store, so that means people will be coming in to buy things to fix unexpected problems. No matter if the customer is in a good mood or bad, treat all of them with respect. If you are having a bad day, keep it to yourself.
If you do something wrong, man up and admit it. Make the effort to correct the problem, and try to make your supervisor see that you care about your job.
Hardware stores = weekend projects. Expect to work weekends while your buddies are fishing. Save your fishing for days off when most others have to work. Weekdays = better fishing. Just ask any of us that have already retired!