Before playing songs with chords, practice each part separately. First practice the melody of the song by itself (right hand part), then practice the chords by themselves (left hand part). Once you have learned both parts, play them together.
Practice slowly and evenly, and beat as you play. Before you play, adjust the tempo to a comfortable speed. To do this, press the start button, and the drums will play by themselves. If the drums are too fast, slow the tempo down by adjusting the tempo control.