The main problem with learning accompaniments to songs from pre-written music either score or Tab is that it normally means you learn the accompaniment before you learn to sing the song. This means that it's very difficult to perform the song in a way that is natural to you. One of the most important things about folk music is "the folk process" where singer imparts some variation and often different interpretaion to a song. One of my "bete noirs" is listening to people who will insist on performing from music in a folk environment.

In Classical music things are different, you need to get the music exactly right as you usually have to fit in with many other people. Even in choirs, though, the conductor will usually start saying "Get those heads up out of your copies and eyes on me!" after the first rehearsal or two. On several occasions when I've had an unaccompanied solo recit to sing - conducters have asked me how I manage to get the extra expression into my singing - I just say Folk Singing.

In most folk song, the words must come first. Learn to sing the song, feel the way that it works for you - then try and produce your own accompaniment that enhances what you're already singing.