I'm replacing my sink, and plumbing as well. How does the metal nut come off that is attached to the metal pipe that is behind the wall? The nut has a chip in it and is leaking water when the water drains out of my sink.

It unscrews counterclockwise. You need a wrench or a large Channel Loc plyer. Replace Defective Sink Drain Plumbing - Pipe Disconnected And Leaking (http://www.hammerzone.com/archives/kitchen/sink/drain/02/replace_old.htm)