Once you have the clone stamp tool selected (a stamp icon near the middle of the tool bar), you will decide what you want to replace; and what you want to replace it with. The clone stamp tool takes one part of the picture and pastes it over another like a brush (in that you may use it like a brush). To select the part of the picture you want to replace the other with, press "Alt" while hovering over it. Now go to the area you desire to replace and click as if using a brush to clone stamp that area. The area being replaced will now show the contents of the area you pressed "Alt" over.