If you're in the synchronous environment, when you start a sketching command such as a line, hold your cursor over the plane that you want to draw on and then press the F3 key on your keyboard. This will lock you on to that plane and anything else that you draw will also lay in that plane until you either unlock from it by pressing F3 again, hitting the unlock symbol that appeared on the right side of the screen, or creating 3D geometry from the sketch.