Is it a shower-tub? If so, close the drain and lie down in the tub. Turn on the cold water. Let the cold water run until all of your body is covered except your head (and possibly knees, if it's a short tub).
Is it shower only? Using only cold water, take a shower. Make sure to completely and fully cover your body in soap lather, and then fully rinse it off. Then repeat. Then shampoo your hair - fully lather it up, then fully rinse. Then repeat.