Help me transpose please!

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I have a song in the key of G (if it has an F# then it is the key of G right)? Well the lyrics start on a D - which is too high for me! I can sing it comfortably starting on an F...but I don't know what key that would make the song in. Is there anyway I can figure out what key I need the song in or do I need a really good pianist to figure it out?