Suppose you sing with a wide range of different instruments and accompanists, you should find yourself singing the same song in several distinct keys at different times. So, what's the best key to sing in?
The certain answer is, it depends. It hinges on your voice type. It varies on the type of instruments that are accompanying you. It relies upon on the skills of the instrumentalists. It depends on the song. You get the picture. If you're singing with a group you may not have a choice of key.
Several singers simply buy a piece of sheet music and sing the song in whatever key it's written in. Or sing it in the key utilized by the original recording artist. Or whatever key the accompanist is ready to play it in. None of those keys might be right for you!
Another possibility for getting a song transposed is to contact the music department of your local college or university. Pupils in music theory and composition classes usually need to do transpositions as part of their work. Perhaps one of them would be able and willing to transpose your song and use it like a class assignment.
To pick the right key for your song, first sing it a cappella and away from a piano. Then head over to the piano and determine the note you started on, what the highest and lowest notes are, and the note you ended on. From that you (or your accompanist) should be able to determine what key you sang immediately. That is most likely your best key for that song.
If that comes about to be the key it's written in, fine. If not, find the song transposed. It might also be useful to have versions of the song in several different keys. For example, if your ideal key to sing a song is F#, that is an incredibly difficult key for most instrumentalists. G major and F major are much easier for both guitarists and pianists, and only a half step above or below F#.
It's helpful to be able to sing constant song in several different keys. You may have an ideal key, but one or two others that are also satisfactory so that you can lodge instrumentalists when needed. Happy transposing!