"Silent Night" features two chord shapes which involve a half bar at the 5th fret, using the first finger. This barring technique may be quite difficult at first, but it is extremely important as bar chords feature predominantly in all forms of music. Practice the chord shapes separately, before playing this song.
The Bb note, which occurs as the top note of the chord in bar 18, is located on the first string at the 6th fret.