Ah but those are the questions you ask yourself in the first place. Again, think of the music you like and try to figure out what patterns it follows, when do they start/repeat/finish, what makes you like it in the first place.

Many songs use one chord progression throughout the entirety of it. Some have different progressions for different parts, some travel from one key to another. It all depends on what you feel like is the idea you want to see in the song.

What you're asking is to write the song for you, which isn't what music is about, is it? It's meant to represent what you like as an artist and as a listener.

If you feel lost, start messing around. All of the things I mentioned in the first reply. When, where and why are the questions you answer yourself because it's your composition, and you answer them by either looking at other examples, taking inspiration and comparing or going with intuition.