Sharp:

The function of the sharp is simple, to raise a note by one half step. For instance, if the note F had a sharp sign next to it you would move up one half step thus making it F#. This can get tricky in the case of B# and E# because one half step above B happens to be C, and one half step above E happens to be F. Don’t worry though, even though B# sounds the same as C, we don’t write as C because of reasons we’ll talk about later. Just remember B# sounds the same as C, but it’s not C. The same goes for E# and F.

Flat:

A flat is just the opposite a sharp, it lowers a note by one half step. The same issue we had with B# and E# also applies to Cb and Fb. (sharps and flats are also called accidentals if they appear in the piece but not in the key signature)

Sharps and flats are always placed to the left of a note and on the corresponding line or space.