The world more or less agreed on the same names to simplify talking about these 12 notes in the chromatic scale.
The White Keys
The naming is based on the white keys on a piano. These keys are named A, B, C, D, E, F and G. See the names of the keys above.
The block is not starting at the note A as you might expect, but instead with C. This is owed the reference tone, which is located at the note A. More about this later.
About the note name “H”
In many countries, the note B is called H for historical reasons. If you read the note name H somewhere, just replace it with B. These are just two different names for the same note.
The Black Keys / Sharp and Flat Signs
The black keys have no unique names. Instead, their names are derived from the location of nearby white keys. This is often a source of confusion.
As each black key has two neighbouring white keys, it also has two names.
If you go to the black key on the right of a white key, you add the sharp-sign # to the note name. If you write all 12 note names in sequence you get:
But there is also the second notation using flat-signs ♭. If you go to the black key on the left of a white key, you add the flat-sign ♭ to the note name. If you write all 12 note names in sequence you get:
If you like to learn read and write musical scores, you need to know this notation and its usages in detail. For the start, all you need to know how to locate the right key if you read a note name.
Flats on F and C / Sharps on E and B
Sometimes you will read flat-signs ♭ on the note F and C. In this case, this means that you move from F to E, or from C to B. The same is true for sharp-signs # on the note E and B. Here you move one key up from E to F or from B to C.