The sharps

A sharp (#) means: one key to the RIGHT. The black key directly right of C is C-sharp, written C#.

That gives the five black keys their first names: C#, D#, F#, G#, A#. There is no black key between E–F or between B–C — those white keys are already direct neighbours.

To find any sharp: find the white key first, then take the black key just right of it.

# = one key to the right

The five black keys, named as sharps.

Practise this in the trainer — it opens with this lesson the first time the concept comes up.