If you want a way to look at. Think of movies, the first thing that a movie has is something that catches the viewers attention. In the same way your song must have that. To catch the ear of the person listening so they don't turn the dial. Next is the verse or the story build up, it's what introduces and build up the song or the movie. Chorus is gonna be the catchy part, everything that somebody is gonna remember. So the song is basically built around the chorus(something you want the listeners to understand or get out of the song). The ending is big and has to be dramatic, it will need to be something that will make the person want to listen to the song again. Most of it comes down to originallity, make the song your own and how its built!!! ;)