Just listen and play along, and try to add something rather than just play the song tune. My instinct, I think, would be to play this song in first position, so work out what key it's in and use a harp in the same key. I might even prefer a tremolo harp personally. And there's no need to be playing all the way through or to do the same thing in every verse! Whenever you accompany a song, you always have to remember that the point of a song is the song, not the harmonica player.