In Brazil, the forms o senhor, a senhora (which correspond to Spanish señor, señora) may not be used as much as their Spanish equivalents, depending on where a speaker is from, their age, etc. In general, the safest way to address someone is to use você. Even if a native speaker might use o senhor/a senhora in the same context, você is becoming more widely used and is generally accepted, except in very formal situations in which tourists would not often found themselves (such as talking to a state governor).
Sound Brazilian! Brazilians normally use first names, not last names. If you want to be formal, you may address someone as Doutor(a), Seu or Dona, but those are followed by the person’s first name. Lists of names are still alphabetized by first name. In informal conversations, Brazilians even refer to the president by his or her first name!
English: Ms. Smith, your car is the white Passat.
Spanish: Señora García, su carro es el Passat blanco.
Portuguese: Dona Cristina, o seu carro é o Passat branco.