Water not flowing through shower head
	
	
		I have moved into a new flat and my shower is not working.  There is a cylinder-shaped part between the two taps that I can move up and down (presumably moving it in one direction is supposed to activate the shower) but the water continues to pour out of the bath taps.  What do I need to do to get the water to flow upwards through the shower head?  Is there a problem with my water pressure?
I am a complete novice to household plumbing, so full explanations of all technical aspects will be much appreciated!  
Many thanks.