Answer:
It depends on what you like - some people may suggest a pop song when you are a metalhead or would tell you to listen to something such as AC/DC when you like classical. Go for new songs (or old songs that you don't have) by bands that you like, or those in the same genre. Many sites will have links to similar bands and artists. Try some teaser-tracks and see if you like any of those enough to go for the whole thing.