Just to summarise some of the responses to the original question.

Obviously listen to all the wonderful singers and ballads mentioned on these threads.

To start with perhaps choose some of the couplet ballads with plenty of refrain and repeats so folks can join in. I started with The Cruel Mother, The Two Sisters and The Maid and the Palmer, 3 very powerful, but easy to sing ballads.

( a little tip...avoid the obscure ones with only a few versions)

I love all of the really popular ballads, listening and singing.

I endorse all Brian wrote above.