It is important not only to learn the chords, but also to keep the rythm. D means that you are strum down over the strings, while U means that you are struming up over the strings. Try to follow the rythm. It is a really good idea to start really slow, when you master the shifts in slow tempo it is easy to make it faster, but don't speed up too fast. Listen to the tempo 80 to hear how it should be, but you should start slower than that.
Be sure that you really can follow before you increase the speed.
Play following chords
The chords are repeated 4 times
Melodies