According to the top answer from Word order with "so" and "also", "so" is never used at the start of a sentence as an interjection.
That said, in Pokemon Schwarze Edition, this is said at some point:
So, dein Pokemon ist wieder putzmunter!
AFAIU, "so" should have been "also", with the sentence meaning "Well, your pokemon are lively again!". Is this correct?