Green Sleeves introduces the key of G major which has one Sharp - F#. Normally the sharp symbol is not shown when F# is played (but turning on Courtesy Accidentals from the Piano Booster setup/preferences menu will show all accidentals in the key signature). The melody is in the right hand with a simple left hand part.

Hint: You can show Courtesy Accidentals from the PianoBooster setup/preferences menu. But remember to turn them off when you get more skilled.