Many of the melodies you already know will use notes derived from the major scale. This example created from the C major scale is written on the treble staff only. Male singers can sing the same melody one octave lower, at the same pitch as the male singer on the recording (second time through).
This octave displacement and is indicated in written music by the symbol 8vb which tells you that the part is to be sung or played an octave lower than written, or 8va which tells you to sing or play the part an octave higher than written.