I am sure you noticed that each scale was unique and different. They all had different notes (even if some had similar notes in them) and each had a different amount of accidentals whether it be sharps, flats or none!

This is where we need to talk about the circle of fifths and key signatures!

The circle of fiths is literally a circle in which you go up the scales by a 5th. Meaning every 5th note of a scale starts the next part in the circle.

So if we start on C, the fifth note of the C major scale is G, The fifth note of the G major scale is D. And so on. Here is a list.

C-G-D-A-E-B-F#-C#-Ab-Eb-Bb-F and back to C

What this determins is our key signature!

When reading music everything won’t be written out in the key of C (no accidentals) then have accidentals thrown in and taken out as one pleases. There will be a symbol that will help you understand what key you are in before reading or playing the music. This will be written out on the left side of your music right after the clef and before the time signature. What this symbol is, is that is shows how many sharps or flats will be in the key.

A musical Key is the set of notes you are playing it is what notes will be played in the song.

This is from wikipedia: "In musical notation, a key signature is a series of sharp or flat symbols placed on the staff, designating notes that are to be consistently played onesemitone higher or lower than the equivalent natural notes unless otherwise altered with an accidental. Key signatures are generally written immediately after the clef at the beginning of a line of musical notation, although they can appear in other parts of a score...Key signatures are generally used in a score to avoid the complication of having sharp or flat symbols on every instance of certain notes. Each key has an associated key signature that sharpens or flattens the notes which are used in its scale."

To summarize: a key signature is a symbol in the beginning of the song on the staff that tells you what notes will be sharp or flat. It tells you so it doesn't have to write out all the accidentals in the score. This makes the song easier to play. Instead of having to read a ton of sharp notes very fast you will remeber oh it was in this key which means all of these notes are already sharp!

So each Note has it's own Major key. Which means there are technically 21 keys, but once again I am making it only 12 major keys.

Let us go over it first by order of easiness then alphabetical order.

C has no accidentals

G has 1 sharp

D has 2 sharps

A has 3 sharps

E has 4 sharps

B has 5 sharps 

F# has 6 sharps

C# has 7 sharps

Ab has 4 flats

Eb has 3 flats

Bb has 2 flats

And F has 1 flat

2 things to notice. 1.) it is going in the order of fifths! This is essentially the circle of fifths. 

2.) It doesnt no tell what sharps and flats these keys have.

So since there is order in music. There is an order of sharps as well as an order of flats that directly connect to key signatures.

First: The order of shaprs means these sharps show in a specific order as you add more sharps into the keys. This order is a very specific order and it is the order of sharps as they appear in the scales. 

The order is this: F#, C#, G#, D#, A#, E#, B#.

I made up a mnemonic: Fat, Cats, Get, Down, At, Every, Ball.

So this means the key with 1 sharp will have the first sharp which is F#.  They key with 2 sharps will have F# and C#. The key with 3 sharps will have F#, C#, and G#.

Since there is an order of sharps there must be an order of flats.

The order of flats is the order of sharps backwards. Bb, Eb, Ab, Db, Gb, Cb, Fb

If you noticed after the sharps the lfats when from high to low and this is why.

So they key with one flat will have the first flat which is Bb. The key with two flats will have the first two flats which are Bb and Eb. Same as before. You get the idea.

So here is that list again with the sharps and flats the keys have:

C has no accidentals

G has 1 sharp, F#

D has 2 sharps, F# & C#

A has 3 sharps, F#, C#, & G#

E has 4 sharps, F#, C#, G#, & D#

B has 5 sharps, F#, C#, G#, D#, & A#

F# has 6 sharps, F#, C#, G#, D#, A#, & E#

C# has 7 sharps, F#, C#, G#, D#, A#, E#, & B#

Ab has 4 flats, Bb, Eb, Ab, & Db

Eb has 3 flats, Bb, Eb, & Ab

Bb has 2 flats, Bb, & Eb

And F has 1 flat, Bb

Also some other very helpful hints:

When looking for a Sharp key signature just move up a half step from the last (farthest to the right) sharp.

As in if you have three sharps, F#, C#, G# the last sharp is G# and it will be the most right in the key signature. So a half step up from G#is A so the key with three sharps is A major.

 For flats you just look at the key signature before your farthest to the right flat. EX: if you have three flats look back one key signature and see E^b Major.

First Here is the circle of fiths picture:

Now since there is an order of sharps and flats for accidentals you need ot know how and where to put this symbol/order on the staff. So what you do is that after you put the Clef you then put the Key Signature and after the key signature you put the time signature. This is all before the song Even begins.

So since you have the order of sharps and flats you would need to know what key you were going to write the piece in. So you pick the key whether it has flats or sharps. Then you put the sharps on the staff on the line or space of the note/letter it would be on. 

Example if I am in D. I have F# and C# So I would put a sharp on the 5th line, and in the 3rd space. To show I was in D Major. Now You might be asking by not the first space for F. I honestly don't know. I think so it is easier to read so you don't have to jump around counting the sharps or flats that it is in an order that is angled. I will do sharp keys first then Flat keys so you can see!