An accidental in music is not something you do by mistake. It is commonly refered to as the black notes on piano. (Even though for all cases it isn't true.)

An accidental is a symbol that you attack to a note/letter.

There are 3 symbols we associate with accidentals.

The Sharp.

The Flat

And The Natural.

First off these symbols are added on the upper right of the note. When writing it out. But when you write it in music it is on the left side. I will show examples later on.

The Sharp is a symbol that looks exactl like the hashtag/number sign: #

It raises the note by a half step. (Example #1)

The Flat is a symbol that looks exactly like a lower case b. It lowers the note by a half step. (Example #2)

The natural is a symbol that is kind of odd to explain. It is an angled square that has a line going up on the left side and a line that is going down on the right side. The natural just makes the note natural. So if you are coming from a sharp and want to play the letter note again. So going from Bb - B(natural). You need the symbol or if you write just a B with no natural it will be played as a Bb again because accidentals carry over until the measure ends or you add a natural. I will show examples of this as well. (Example #3)

Every note can have a sharp, flat, and natural. (Example #4)

Like so:

Now when you write an acidental in music it is on the left of the note. Like this: (Example #1)

Here are all the notes on a staff with sharps, flats, and naturals. I will use all quarter notes. (Example #2)