The correct terms would be "ragazza" for girlfriend and "fidanzata" for fiancée.
However, nowadays we almost lost this distinction and the word "fidanzata" has become just "girlfriend". For this reason both "ragazza/o" and "fidanzata/o" are common for girlfriend/boyfriend. We also use to say "Sono fidanzato/a" when we are "just" in a relationship, with no marriage on the way.
The drawback is that we've lost the word for fiancée, which is still "fidanzato/a" but this creates a bit of confusion :)