"Clair de Lune" is one of the most recognisable pieces of the piano repertoire (thanks to many television adverts) and was written by French composer Claude Debussy. It is the 3rd movement from his Suite Bergamasque for solo piano, and is one of the most atmospheric pieces around. You should play it rubato (with a gently impulsive rather than a strict tempo), and let the musical ideas flow and grow. The complete "Suite Bergamasque" for piano, consisting of four movements in total, is available in a number of editions including these from Sheet Music Plus and The Music Room.