This new topic is: Italian lessons. I will write some basic Italian for beginners, but anybody can ask questions and I will be pleased to answer.
So, let's start, Lesson number One:
Ciao: Hello/Hi; very informal, but also common among friends and so on... You can use it either when you arrive or when you leave.
Salve: hello; a bit less informal, but anyway not a formal way of greeting.
Buongiorno/ Buon giorno: Good morning; You can use it until noon, as in English. You can say Buongiorno when you start a conversation, when you arrive and (in formal context) you can say so even when you're leaving. My Granma used to say that it was more polite than "arrivederci". Actually, nobody would say that "arrivederci" is unpolite!
Buon pomeriggio: Good afternoon; I think that a common grammar would tell you that you can use it when you arrive or starting a conversation after noon, but it is quite unusual to use it like this. If you really want to say so, and not give the impression of being a foreigner, use it before leaving, is a good and polite way to wish a nice day!
Buona sera/Buonasera: You can start using it after noon. As Buongiorno, is mainly use when you arrive, and if you want to be very polite you can say so when you're leaving (see Buongiorno).
Buona notte/Buonanotte: Good night; mainly used before going to bed, or very late at night.