(SPOILER)Do you want the very first time you see it or the first time you see it and say "Whoa, an airship!"? The former is after defeating Tros, where Rikku is in your party as ????. It's "the big prize" that the Al Bhed are salvaging. However, at that point it isn't clear what it is (at least, it wasn't to me). The airship in its restored glory is first seen at Home. Are we allowed two guesses? If not, I'll say at the start near the Salvage Ship, since that's the more literal answer.