Practice practice practice. It will get easier. I also found it helped me to listen to a song I wanted to learn over and over, listening to the chord changes and what word the chord changes came on. Then when I practiced, I would start slowly, making sure I changed chords on the right word, and gradually gained speed. Kind of a weird method maybe, but it always seemed to work for me. Some good three chord songs to start with might be "Can't ya see" by Marshall Tucker (d, c, g) and Sweet Home Alabama (d,c g,) Both are pretty easy to sing along with too.