Ask the vendor for what you and the weight or count of what you want to buy. They select the produce for you. They then weigh the produce and give you a total to pay.
if you want to purchase individual bakery items such as dinner rolls or produce sold by weight such as carrots you would select your items and then put it in a plastic bag. There will be a scale near each department. Put the bagged item on the scale and enter the code that shows on the sign by the item i.e. Carrots, Weight, #. The machine/scale will then spit out a label and you affix it to the bag. The cashier then scans this code.
Grocery stores don't automatically give bags. You may need to ask for them.