Fretboard Diagram

The diagram below shows all the natural notes on the fretboard of the guitar.  Not indicated are the accidentals.  Accidentals are notes preceded by a sharp (#) or a flat (b) symbol.  A sharp raises the pitch of a note 1/2 step, or 1 fret on the guitar.  Therefore an F# would be played on the fret between the F and the G notes.  A flat lowers the pitch of a note 1/2 step.  Therefore an Eb would be played on the fret between the D and the E notes.