"I am fine, thanks."
Translation:Sto bene, grazie.
No, "io sono bene" isn't really used, but "è bene" or "è un bene" means "it's a good thing".
In English we know the "to be" verb, e.g: am, are, is. But in Italian, we know that there are many kinds of "to be" not only essere, but also stare, and avere, e.g: sto bene (I'm fine), ho fame (I'm hungry), etc.
How do we know when we should use one of them? I mean, is there any standard formula to use it?
Mille grazie per le vostre spegiazioni.^^b
Sorry, unfortunately I don't know any rule to it; it's common for a verb to have multiple meanings, and that means that direct translations get rarer the further the languages are. For instance the Collins dictionary gives 35 meanings for "get" (and only because it considers its prepositional phrases as different entries): well, most of them have their own different translation in Italian.
As far as the difference between essere and stare goes, this Italian post clarifies the difference when describing positions, but there is much more to be said; I think it's best to learn the specific contexts one by one, as you meet them.
This is the reason why a native speaker will always know that we're not one of his :-(.
How am I meant to remain undetected once I immigrate illegally in Italy now? Damn!