This is a beautiful old carol with four possible vocal parts. In fact anyone can sing any of the parts that suit – perhaps choosing an octave that is comfortable. It has a musical twist on the last part of every verse that is just lovely. The best way to learn it is to listen to the arrangement as a whole, then the individual parts, pick one you like that’s in the right register for you, sing it a bunch of times in a row until you’re really familiar with it, then try singing your part along with the recording of all the parts.
To really develop your ability to sing harmonies, you can try also putting your part alongside one of the other individual parts. There is little more magical sounding than voices in harmony. Here are the lyrics.