"The Wedding in My Dream" has two flat b's, which are 7(si) and 3(mi) respectively. That is to say, all 7' s and 3' s on the piano score should be flat by half a note, that is, black keys to the left of the white keys of 7' s and 3' s, and the special key of "ヰヰ" with a temporary flat tone is marked in the upper left corner. This piece only restores a 3, and most of the 7' s and 3' s are black keys. Apart from these, all the other notes are played with white keys, except those marked with temporary ups and downs, but this piece has nothing.
I don't know if you understand.
I suggest you search some basic knowledge of music theory on the internet and learn it first. I used to teach myself! Wish you success!