我執左首歌既所有音(note), 點先知道首係乜調(key)

you can sing the song once, find the 'do' note.
the key is in the 'do'-major.
Eg.
if the do note is E, then the song is in E-major
If it is in minor, just change the major into its relative minor.
Eg.
if the key is in e-major, the relative minor is C sharp minor.