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)

F=Fret S=String

F S

0 A

1 A# or Bb

2 B

3 C

4 C# or Db

5 D

6 D# or Eb

7 E

8 F

9 F# or Gb

10 G

11 G# or Ab

12 A

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

F S

0 E

1 F

2 F# or Gb

3 G

4 G# or Ab

5 A

6...