The standard shower has two knobs, one for hot water and one for cold water or one knob for both that mixes the water. However, some showers that exist within a bathtub may have three knobs. In addition to the hot and cold knobs, there is a knob that acts as a diverter. In many showers the diverter is a plunger on the tub spout that operates by being pulled. In a shower with three knobs, the diverter is the middle knob.
Turn the left-most knob counterclockwise until the water flow is moderately strong. This will begin the flow of hot water. Allow the water to run until it is fully hot. At this point the water will flow from the tub spout.
Engage the cold water knob, which is the right-most knob. Turn it counterclockwise with one hand and place your other hand under the stream to test the temperature of the water. Adjust the cold water knob until the water is the desired temperature.
Grasp the middle of the three knobs and turn it counterclockwise. The middle knob is the diverter and it redirects the water to the shower-head. Turn the middle knob until it stops but do not force it.