You really don't want to think "key" when arranging for vocalists, rather you think "range."
If you have a standard female, you want to keep her between the "G" below middle "C" to the C above middle "C". If you have what people would call a contralto, a girl with a low husky voice, you want to keep her between an "F" below middle "C" to the "A" above middle "C".
If your singer has a bright high voice, you should keep her between "A" below middle "C" to "D" on the fourth line of the treble clef.
Your male ranges (using treble clef, accounting for the fact the male voice sounds an octave below what is written on the treble clef) are as follows:
Standard baritone range (Frank Sinatra) is Bb below middle "C" to the Eb near the top of the treble clef. Your rock and roll screamers go from maybe the D above middle "C" to the "G" sitting on the very top of the staff.
Country basses sing between the G below middle "C" to the D on the fourth line of the treble clef.
Fit the melody of the song into the appropriate range, then determine what key it is from there.
The above ranges are what you would call workable ranges. Usually, each vocal type will be able to sing lower and higher than the listed ranges. Your singer can help you with these extra notes. It's not a good idea to make your singer sing at the limits of his or her range, though.