Forgive me if you already knew this, but.........
There are 2 basic styles of harmonica: chromatic and diatonic.
The chromatic has a "press button" in the side which helps you to reach all the sharps and flats. So, like a piano, you can play every note, and you can play a scale starting at any note and you will be able to include any sharps or flats. With practice, that is.
A diatonic harmonica is set up to mostly play just the notes of one scale. With practice, you can force the instrument to play some "out of scale" notes, which is called note bending, but you cant play every note including all sharps and flats.
Where it becomes difficult is when you try to play along with either other musicians, or with a record. Because apart from the difficulty of hitting the right notes, if you go for a diatonic harmonica, you need to have one in the right musical key. Diatonic harmonicas come in different keys (and C is the easiest to get your head round at the start), but if you do get hooked on harmonica playing, you may well eventually need some more harmonicas in different keys, as well as the C one......
I have now used up all my harmonica wisdom, so I will shut up now!