Without seeing the sheet music you have, I can't really tell you. Sometimes it's written which position to be in, other times you just have to use logic. Which C does it sound like and where is the easiest place on the fretboard to play it?

Since you said you know piano and can read music, this part should make sense to you. Middle C on the guitar is on the B string, first fret. Guitar sheet music written in the treble clef is usually written one octave higher than the actual pitch. So the pitch you play, is an octave lower than what is actually written.