(For visual support, please check the video attached, as well as the diagrams with it.)
Western Music is divided into 12 equal tones. The distance between two notes without a gap is known as a "half step". The distance between two notes with the gap of one note in between is known as a "whole step".
The way notes have been named, is with the help of English alphabets: A BC D EF G. Notice the fact that BC and EF have no gaps. It is because they are naturally half steps, and it is because the naming has done after the shape of the piano. Let's see :
Do you see those black notes? Those are the notes in between—the distance between C and the black note after it is a half step. The distance between C to D is a whole step.
Notice that E and F, as well as B and C, are naturally half notes, as they are adjacent to each other.
Now how do we name the black notes? Let's see a basic thing: when a note moves towards its adjacent note, it sharpens. E.g. The black key in front of C is C#. ("#" is the symbol used for sharp.)
Similarly, when a note moves behind to another adjacent note, it flattens. E.g. The black key behind D is Db ("b" is the symbol used for flat)
Now lets put it to practical use. Let's take one string, the first thickest string. When played open, it is E. Let us see how it is named in sharps. You will notice the complete 12 tones on just one string. Imagine, you could play so many melodies on just one string!
It is the string in sharps. E to F and B to C are naturally occurring semitones.
Let's look at the same string in flats.
You will notice that each black note has two names, depending on the context of the other two alphabet notes surrounding above and below it.
Your exercise is to find out all these notes on all strings. I have mentioned how guitars are tuned in the open. If your guitar is not tuned, use a tuner and tune each string according to the notes written above, e.g. E is the thickest string, A is second thickest, D is third thickest, so on and so forth. Good luck, and comment below if you were able to do this on your guitar! :)