Is it legal for me to commericially sell guitar lessons of famous songs on DVD?

While it  is not illegal to show someone how to play a famous song, if you record your lesson and attempt to sell it, your lesson will include you playing the famous song, which can be considered a performance of the song, which is an infringement of the copyright owned by the person that wrote the song.  So while you think you are selling your instruction, you would also be selling your playing of the song, which is a copyright infringement, unless you have a license from the owner of the song.  You may want to contact the major licensing companies,  BMI,  ASCAP, and Harry Fox Agency to determine the costs for the license.