‘Joy’ in music can refer to feelings of love or hope. This group of emotions are generally characterised by positive, uplifting feelings. In order to convey these positive feelings, focus on a stable musical material, with a tonality that is predominantly major and consonant. High energy is often useful too, but not always necessary.
The stability will provide a sense of comfort and dependability for the listener. A consonant tonality performs a similar role. Both the stability and consonance will allow the major tonality to come through clearly. The energy level will depend on the overall contour of the song, but a high energy level can can also assist in drawing the listener’s attention. A high energy level will also indicate to the listener that a particular section of music is particularly important (which also helps make it more memorable).