At the car wash, lots of dirty cars gets cleaned everyday. The staff don't care how dirty your car is, as they just want to make you happy and fill their pockets. It is the same thing whether your car is filthy or not. They just want to give you the best service that they could provide.
The game is played via mouse and you need to interact with the surroundings to play it. Grab those hose and wet the car first, so that the dirt would loosen up. Put some soap to clean the car. Rinse it and put some wax on it to make it shine like new.