Once you know the highest note you can sing, the answer is
as close or equal to the highest note you can sing, but no higher!
For example, the highest note I can sing comfortably is D above middle C. Now, we need a song. How about "By The Light Of The Silvery Moon"?
- The song is written in the key of C.
- The highest note in the song is F ("Your silvery beams will bring love dreams...")
- F is the fourth note in the C major scale (C-D-E-F), so...whatever key I sing the song in, the highest note will be the fourth note of the scale.
- Since D is the highest note I can sing, I look for the key in which the fourth note of the scale is D.
- That scale is A! (A-B-C#-D)
I'd begin by seeing how the song works in the key of A.
- If the highest note is a passing note, I might be able to choose my key based on the second highest note.
- If a song lives near its highest note, that is, most of the notes are high or there is a long passage of high note, I might have to pitch the song in a lower key (G or F).