Bathtubs that function both as a bathtub and shower tub have a shower diverter valve built into them that allows you to switch the water flow from the tub faucet to the shower head. These valves change the flow of water by blocking it from the tub's faucet and forcing the water upward and out the shower head. They are usually operated by the simple turn of a valve or push of a plunger.
Locate the shower diverter valve and determine whether you have a rotating valve type or gate type valve. A rotating valve diverter takes the form of a handle that turns from side to side and is usually located somewhere in between the tub faucet and shower head. A gate type diverter usually takes the form of a plunger that is located on top of the tub faucet.
Turn on the bathtub's faucet. Water should now be flowing from the faucet into the bathtub.
Turn the valve handle in its free direction if you have a rotating valve type diverter. If you have a gate type diverter, pull up on the plunger on top of the tub's faucet. The water should now cease to come out of the bathtub faucet and should instead come out of the shower head.