You now know enough notes to play the D major scale. This scale contains two sharp notes – F# and C#. The sharp signs for these notes can be written at the beginning of the staff. The second sharp sign (written in the C space) tells you to play each C note as C#. These two sharp signs are the key signature for D major.
This song is written here in the key of D major. The sharp sign after the clef also applies to C notes below the staff, so make sure you play C# notes in bars 8 and 14.
You can now play the song on page 22 of Supplementary Songbook B.