When using the clone stamp, you have to first figure out your source point (the point you want to use as the source). To do this, hold the ALT key, the mouse cursor will turn into a crosshair, then left click on your source point and left go of the alt key and mouse button. Now place your clone brush (mouse cursor) over the area you want to "cover up", then click your mouse and start painting away.
Remember to always work with a copy layer, not the original. If you make a major mistake, you always have the original as a backup. Also, try doing a search on google like "+photoshop+clone+stamp+tutorial". This may help you out more indepth than my quick reply. Good luck.