No question is stupid and this is a fascinating question! Although it cannot tell you what key you are singing in it can tell you what note you are singing. I have have never thought of this before so I just collected my Boss TU-80 out of the car and if you sing a note it will tell you what it is (and show you if you are sharp or flat ).
Because the vocal range of songs do vary I find that although I sing the majority of songs in G, I have some songs that I have to sing in A, Bb, C, D, E, or F to comfortably suit my voice.
So there is not an easy answer to finding the right key, but I suggest that you record yourself singing a song and then, using a capo, try it in different keys and listen to the results as to which sound best and which you find comfortable with.