There are many instructional websites online that have a vast array of information on sailing. Of course the best education is done with on-hands experience. In my opinion one of the best training schools is H.O.R.C. (in Greek Π.Ο.Ι.Α.Θ.). This excellent school in Kastella (close to Piraeus) has good sailboats, excellent facilities and good website (click on the image).