For guitar players, it's important to get to know the notes on the staff below the bottom of the Treble Clef. Usually, guitar music is not written using a Bass Clef, so we need to get familiar with the ledger line notes starting with Middle C and going down to F.
Just like on the treble clef staff lines and spaces, we get the repeating patterns of F A C E (face) and E G B D F (Every Good Boy Does Fine), except for the pattern is switched! Below the staff the lines are F A C E and the spaces are E G B D F.
Here are diagrams for notes below the treble clef staff. Click the "fretboard" button to see the fretboard.