I would start with what I call the musical clock which is the 12 notes in a full octave rolled into a circle to replicate a clock (also with 12 points for each of the hours) This makes it easier to picture.
Starting at the top with A can work our way around the clock with the musical alphabet for which I use this rhyme.
The Musical Alphabet is A to G and Everything has a Sharp (#) except B and E
So if you can now picture that clock and start from the Top (A) then the A String on the Guitar is the note of A (Obvious I Know), the 1st fret will be A#, the 2nd Fret B, and so on (see below)
1 A# or Bb
4 C# or Db
6 D# or Eb
9 F# or Gb
11 G# or Ab
So, reading through this is reading the musical alphabet as above
Once you know the musical alphabet and how it translates to the A String you can then transpose this to the other strings so for the E String (which will count for Thin E and Thick E
2 F# or Gb
4 G# or Ab