First, click the arrow to choose an instrument. Then click on one of the ovals to set it to play a note. Having trouble making a beat? Try using the presets on the right side to quickly fill out the rhythm. You can also drag on the ovals to change the pitch of each note. 

Your track can have up to five instruments, each with their own melodies. Once you have five instruments set up, click on the spinning CD to listen to a performance of your song! Keep working on your chart-topper until you're happy with it.