Again let us go back to The Sound of Music. When she teaches the kids to sing she starts by teaching the Do re mi fa sol la ti do. Also called solfeggio (sp). Each of those is assigned it’s own spot on the major scale. Then she makes up a song in these terms. Do sol fa mi re do re, do sol fa mi do re do. The kids complain that it dosen’t mean anything so she then puts words to the notes and makes a song. When you know the notes to sing, you can sing most anything.
You can do this process in reverse.
Take Mary had a Little Lamb and figure out the solfeggio words (do re mi) for that song. Then what ever note ‘do’ (or if I were singing d’oh!) ends up being, that is the key that you are singing Mary had a Little Lamb.
I would write out the solfeggio for Mary had a Little Lamb but I think you should do it as an assignment so you can learn how to find the key of a song.