The trick when picking music is to read music well enough to know whether the song’s notes are within your range. You don’t have to know everything about what’s on the page; you just need to know enough to discern the difference between the right key for you and the wrong key.
When a singer says, “I need this song in a higher key,” he means that he wants the notes of the song higher. After looking at the song, you may also determine that you want the notes to be lower — that is, you need to sing the song in a lower key.
A song’s key just means that the song is written with one note as the central note, or tonic. That central note is the name of the key. If Middle C is the central note, you regularly return to Middle C in the song.
You don’t have to know all about reading music, but you need to know whether you want the song four steps higher or two steps higher. Wanting the song in a higher key means you want the song to sit higher in your range.
Sometimes singers ask, “What’s my key?” when they actually mean, “What’s my range?” Not every song that has Middle C as the central note has the same range. You want to be able to describe your range and to know whether you need the song in a higher or lower key.