The bus company is ATC and has several information booths around the city, including on the square in front of the train station.
Bus tickets can be bought in tabaccai (small tobacco shops all over the city). An 8-ride bus ticket called Citypass is also available and can be shared by a group.
It is generally impossible to hail a cab on the street. There are several taxi ranks around the city (station, etc.) where you can take one. Otherwise, you will have to phone a cab company.