in informal restaurants, locals addressing a server will normally say moço for males and moça for a females - moço was a general term, used decades ago, to refer to young males in everyday situations, it eventually evolved in the common term used for addressing servers of any age category.

in more formal establishments, garçon for males, and garçonete, for females is also an appropriate,  while the preferred way would be calling the attention of the servers with a polite or subtle gesture, or waiting for them to come to the table.

inappropriate ways to address servers in restaurants is by whistling, snapping fingers, or making 'psssst' noises - while this happens often in the city, it is not considered polite, and more so if done by a foreign visitor.