The procedure is to approach a shopkeeper and say "buon giorno" (good day) or "buona sera" (good evening) followed by saying exactly what you'd like to buy. The shopkeeper may motion for you to help yourself, or may give you a bag, which you can take as permission to help yourself. If not, then you'll have to deal with weights and/or numbers. "Un chilo di mele, per favore." - A kilo (2.2 pounds) of apples, please. "Tre pere, per favore" - Three pears, please. If you want ripe ones to eat today, you might append "per mangiare oggi, per favore." - to eat today, please.
If you don't know the language, you can always point to the bin and use hand signals. Be aware that they're different in Italy than they are in many places around the world. "One" is signified by raising the thumb, as if you were hitchhiking. The number "two" uses the thumb and index finger. Then you just add fingers till you run out. And just because you have permission to select your own potatoes, doesn't mean they want to see you rooting through the bin tossing your rejects hither and yon. You're expected to carefully select and touch only those objects you wish to buy, unless there's obviously something wrong with them. It's all about hygiene.