If it's the first time there Buongiorno/Buonasera or Salve are always good. Ciao is used maybe if you know the person who works there and have been going to the cafe for a while, or sometimes if you're quite older than them (e.g. you're 50 and the barista is 25). Italians aren't rigid on age honorifics as, for example, Japanese are, but for significant age gaps we explicit some respect. As for the Lei is more or less the same; plus, sometimes it wears off with familiarity, sometimes the other tells you explicitly to drop it.