In standard notation, middle C is shown on the first line below the staff in the treble clef. Guitarists are taught that middle C is played on the third fret of the "A" string, which is the string beside the low "E" string. However, this note on the guitar is not actually middle C. Guitar is played an octave lower than it should in standard notation, because that makes the notes in the first five frets fall nicely within the treble clef. Middle C is actually played on the "B" string, on the second fret.
Middle C is named "C4" and the note number is 60. When looking at MIDI note names and numbers in relation to middle C, the six open strings on guitar are as follows:
Low "E" -- E3, 52
"A" -- A3, 57
"D" -- D4, 62
"G" -- G4, 67
"B" -- B4, 71
High "E" -- E5, 76
All other notes can be extrapolated from these numbers. Each half-step is one increment in the note name and number.