If you have a cathedral or vaulted ceiling, you can still use tongue-and-groove boards on it. The application will be the same, except that you have to match the angle of the ceiling and cut the wood accordingly. Use an angle finder to be more accurate when doing so.
To give you an idea, a regular, flat ceiling will have an angle of 90 degrees, so you won’t have to angle-cut the wood. On a vaulted ceiling, however, the angle will be obtuse. When the angles differ from one part of the room to another, work on each board individually, cutting it at the angle of the section where it will go.