I am just starting guitar and have the same question. I created this mapping diagram to flesh out my understanding (trying to leverage my little understanding of piano). Can someone please confirm this is correct, and if not let me know how it is wrong.

The tone of middle c on the piano (C4), is annotated on the ledger line between the treble and bass cleff on the grand staff. That same tone (C4) is played on the guitar at the first fret of the B string (or 5th on G, or 10th on D, or 15th on A, or 20th on low E), but it is noted on the treble cleff (in guitar notation) as the C in FACE.