When I first learned the major scale on the guitar with standard tuning, I was reminded of Do Re Mi from the Sound of Music and that song is apparently in the key of C when I look for sheet music. However, when I begin the scale with the root note (Do) on C, it doesn't right at all when played along with the song. However, when I move the root note down to A sharp, it sounds perfect. Am I interpreting something small wrong that I haven't realised or what? Any help would be great, thanks.
It happens lots of times! The music is written in C major, usually, but the film version of the song is sung in Bb (A#). For various reasons that don't need airing here and now. So, yes, play the tune starting on Bb, so Bb is actually doh, and it will work. Basically, every note will be a tone lower than where you normally play it. Very easy to move down two frets if you've learnt it with no open strings.
The song refers to tonic sol-fa which can be moved to any key, making the root note doh. As it happens, in C only, it could also be deemed to be in solfege, where doh is always C. Try singing this in Bb using the solfege names, and it's a bit of a challenge!