Sorry to hear from your loss
Did the store give you a 1 week guarantee for the fish?
Can you go back and ask your money back? Or get new fish @ no costs?
The best thing to do is to get everything out. Throw the plants. Clean all decoration in hot water. Rinse the gravel in hot water as much as you can.
Clean the tank with aquarium salt and hot water, rinse it thoroughly to remove the salt residues, as goldfish are freshwater fish and salt should only be used for treatment.
Sun also kills germs.
After all this is done, set up your tank and let it cycle before adding new fish.