Written sheet music is often published in the key signature the song was originally recorded or written in.
It is common practice to transpose songs to a comfortable key when singing jazz standards and popular music but not in classical music or musicals.
The first thing to do is to find the lowest and highest notes you can sing comfortably.
Remember that depending on the time of day and your health it might change so give yourself some leeway.

You should also think about the range in which your voice sounds the best.
Find the lowest and highest notes of the song and see if they fit in your vocal range.
If they don’t, count how many semitones you need to move up or down so that the song fits comfortably within your range.
This will point you towards the best key.
Now, try singing the song in the keys that fit your range comfortably and choose the one that sounds and feels the best. This is where iReal Pro can be helpful with its transposition feature.
Some songs span very wide ranges and also, some singers don’t have a very wide vocal range (like for example Billy Holiday) so it is sometimes necessary to change octave of parts of a melody.
This can be practiced by singing songs where some parts of the melody is above or below your range.
When you get to these parts choose a musical place to switch the octave of the melody so it moves into a comfortable range for you.
This will come very handy during jam sessions or other situations where changing the key of the song is not possible.
Use the practice feature in iReal Pro to experience singing a song in all 12 keys.