Question: So does that mean the first syllable is ALWAYS stressed in Italian?
Answer: No, it doesn't. I had in mind the following: the stress in Italian usually preserves its postion in Latin. Actually, there are no rules for the stress position in Italian, but there are such rules for Latin. E.g., the last sylable cannot be stressed (for more precisity, enclitic additions (-quo, -que) are considered to be the last sylables).
About Giovanni: It is "jo-VAHN-nee" in Italian since it was "io-HAN-nes" (Iohannes) in Latin.