A mode is a selection of tones, arranged in stepwise motion, that form the basic tonality of a composition. It is moveable, that is, it can be used beginning on any note. A key, is a scale consisting of a selection of tones based on specific notes and beginning on a specific note. Therefore the "Do Re Mi " (or "Do a deer") song can be considered to be based upon a mode and can begin with any note in the music alphabet, while the key of (for example) A flat major must begin upon the note A flat. If you have the means, get a competent voice instructor. I once had a square dance caller come to me for voice lessons because he was considered "tone deaf" by those who attended dances where he called, and he could not match pitch at all. I had him come twice per week for over a year and I assisted him such that he could sing along with the music recordings they danced to. It was not easy, but quite an accomplishment for him as well as for me. There is hope for the less musically gifted! That hope is found in proper training.