The melody is the simplest part of the song. Consider your country`s national anthem. Unless you're from Spain, it should have lyrics. Those lyrics are not the melody, but the pitch (high or low) and timing (long or short) that you sing along with is the melody. It is, roughly speaking, the part you hum or sing along with. Consider Twinkle Twinkle Little Star. The bit that you would recognize as that song is the melody. Basically, if one person can do it and still have it be recognized as that song (as opposed to another one), there's a pretty good chance it's the melody. Of course with more complicated music, the distinction between melody, counter-melody and accompaniment can be blurred a touch.