How do you put different parts of the song together?

It is not clear exactly what you are asking, but of course if you are singing a song with multiple verses, you sing the first verse first (sometimes followed by a refrain), then the second vers, and however many verses there are. For most songs, the verses will have the same, or almost the same, musical notes.

If you are asking about adding harmony parts to a melody, a full answer is not going to fit here. You could easily spend a semester or more studying chord structure in order to know how to harmonize simple songs.