As explained by They Might be Giants in this catchy tune (and adorable video).

If you were wondering what carbon, nitrogen (and oxygen) have to do with turning hydrogen into helium, the process is known as the CNO cycle.   The CNO cycle isn't actually the most important process for fusing hydrogen to helium in the Sun (that would be the proton-proton chain).  Although some of the helium in the Sun is created through this reaction, it plays a much more significant role in stars more massive than our Sun.