We mentioned earlier that there is no space between the notes B and C, and between E and F in the musical alphabet. This applies directly to the notes on the guitar. Look at the open sixth string - the "E string" (named because that is the letter of the open string). As you can see, the note F appears on the first fret - directly following the note E. As you continue up the string, you'll see that the notes F and G are separated by a blank fret, as are G and A, as well as A and B. This is when we encounter the other exception - the note C directly follows B on the fretboard. Between C and D, there is a blank fret, as there also is between D and E. You've now worked your way up to the 12th fret of the guitar (often marked by double dots on the fretboard), and are back at the letter E again.
This counting process is exactly the same on the fifth string (the "A string") as it was on the E string. Try working up the string without looking at the diagram, saying each letter name out loud until you've reached the 12th fret, at which point you should be back at the letter A again. Remember: no space between B and C, and also between E and F.