#1
How do you know what notes to sing over a melody or how it should sound
#2
Some people know instinctively but most people have to work on hearing intonation (whether or not someone is in the right key or singing the right notes, "in tune")

The rythem of the words will have to go along with the music, and you have to match the notes on octaves.

Are you trying to write a song or learn one?