Chords are most commonly used to accompany a melody. Most sheet music has the vocal or instrumental melody written on the staff, with the chord symbols written above it. Once you are comfortable changing between a few chords and playing some simple progressions, it is important to learn the chords to as many songs as possible. This will help you get a feeling for common progressions and ways of playing chords, as well as giving you lots of things to play when you get together with other musicians.