Teach them about where to find the foods and have them take you to the place where the foods are stocked. Teach that the fresh fruit and vegetables are in a special section. Teach where the cold foods are kept etc. Use this as an opportunity to teach new words for foods. Whenever possible, have them select the food, bag it, and put it the basket. As you are shopping, have them cross off the items on their list which they have put in the basket. This will help them to focus on the task of shopping.
When they get older, you can have them push the cart and have them search for the listed items in the store. You can also teach them to identify who works in the store and how to ask them where the items are. Always, have them bring a list and cross items off. When they get better at all this, you can take them shopping, and give them half of the shopping list to shop on their own. Set up a meeting place, (in front of cashier #1, or aisle #2) to make sure that they can navigate by themselves.