Why does the steering wheel makes a creaking sound when it is tunred which is louder when the car is stationary or moving very slowly?

you are low on steering fluid(maybe a leak) get it checked out before it is too late.

UPDATE: (I can now answer my own question) Actually, the answer has since been discovered to be bad strut mount bearing plates (Toyota Camry). Once the strut mounts (and their bearing plates) were replaced, all has been quiet for the past 8 months. The creaking was similar to the sound of a old wooden ship in the movies (like Pirates of the Caribbean).