Even though you are learning the flute, I think it is worth understanding the piano keyboard, and thinking about key signatures in terms of that. It's worth experimenting with a piano, or an electronic keyboard, or even web app like [this one][1].

![Piano keyboard annotated with note names][2]

The white keys are the 'natural' notes A-G. The black keys are used to play sharps and flats. Notice that B flat is the same piano key as A sharp, and so on. 

There is no B#/Cb or E#/Fb -- that's just one of those things; I suggest getting used to the basics before trying to understand why this is.

The first scale most people learn is C major. That uses just the white keys, with no sharps or flats.

You'll find you can play lots of simple Western melodies using just the white keys. Try *Twinkle Twinkle Little Star* - C,C,G,G,A,A,G  F,F,E,E,D,D,C

That melody begins and ends on C; but more generally it 'feels' as if C is the 'home' note. Try ending on a different note -- somehow the tune feels unresolved. So the melody is 'centred' on C.

So this is *Twinkle Twinkle* in C major because:

 - C feels like the home note
 - It uses only the white keys, which is the C major's *key signature*

Now let's try playing *Twinkle Twinkle* in D. What happens if we just move all the notes up by one white key? D,D,A,A,B,B,A G,G,F,F,E,E,D

Actually it sounds great, until you hit the F. The F sounds wrong. You can tell why, by counting the black keys between notes. When we played that part of the melody in C, it stepped from F to E - that's one semitone because there is no black key between F and E. Now we're stepping from G to F there's a black key inbetween, making it two semitones.

To fix that, play an F# instead. D,D,A,A,B,B,A G,G,F#,F#,E,E,D

What we've just done is called *transposing* - moving a piece of music from one key to another. We've transposed *Twinkle Twinkle* from C major to D major.

*Twinkle Twinkle* doesn't contain the 7th note of the scale, but you can see that the step from A to B is two semitones, and the step from B to C is only one semitone. So to fix that, in D major we play C# instead.

So - to transpose something from C major to D major, move everything up by one note letter, play F# instead of F, and C# instead of C.

When writing music on a stave, it gets messy having to put sharp and flat signs every time a black note is played. Instead they are marked at the beginning of the piece, just after the clef. If we were writing down our version of *Twinkle Twinkle* in D major, we would write the clef, then a sharp sign each on the F and C lines, then the time signature.

It turns out that no two major keys have the same combination of sharps and flats, so a musician will look at those two sharp signs, and say "aha, D major" -- so that notation is called a *key signature*, because it is an easily recognisable indication of what key we are using.

But it's note quite that simple, because we also have minor keys. A minor has no sharps of flats, just like C major. The only way to differentiate is to recognise that the melody is centred on A. Likewise, B minor has the same sharps and flats as D major. 

There are more scales too -- but that's a more advanced topic, to be tackled when you're comfortable with these basics.

All of this is exactly the same on your flute; it's just not as visually obvious as on a piano keyboard.

  [1]: http://www.html5piano.com/
  [2]: https://i.sstatic.net/tk0fP.gif