Sheet music is usually published in the song's original key, but singers often transpose to a key that suits their voice.

 

Find the lowest and highest notes you can sing comfortably, compare them to the song's range, and count the semitones needed to bring the song inside your range. That points you to the best key. Use iReal Pro's transposition to try the keys that fit and pick the one that sounds best. For a melody that is too wide for any one key, switch the octave of the out-of-range phrases at a musical spot.

 

For the full guide with diagrams, see Find the right key for a song on the iReal Pro website.