Excuse me, where is my seat?
POS-to — stress on first syllable. Short and clear.
When boarding and struggling to find your assigned seat number.
'Posto' means seat, place, or spot. 'Dov'è il mio' (where is my) is the simplest way to ask about something belonging to you. Seat numbers on Italian flights are printed on the boarding pass (carta d'imbarco) and are the same system worldwide: row number + letter.
Questo è il posto 14B?
Is this seat 14B?
Confirm a specific seat number with a fellow passenger
Mi scusi, credo che lei sia seduto al mio posto.
Excuse me, I think you are sitting in my seat.
Polite but firm way to reclaim your seat
Posso sedermi qui?
Can I sit here?
Ask before sitting in an unoccupied seat
Italian airline boarding passes use the standard international format: row number followed by letter (A=window left, B=middle left, C=aisle left, D=aisle right, E=middle right, F=window right on wide-body aircraft). On small aircraft the letters go A-C on one side and D-F on the other.