Defocus please help me i suck at nodes

im not the best at nodes and i cant find any tutorials on how to defocus for a animation and how to work the defocus period, so can some one give me a link or tell me please?

http://blenderartists.org/forum/showthread.php?t=72800&highlight=Node+Depth

:smiley: Or just use the defocus-Nodeā€¦ Connect your image and Z-pass with this node and the output.-