If you've ever wondered about what key a song is in, this will help.
First, let me say this. This stuff is much clearer when looking at a piano keyboard. Not that you need to know how to play piano, you don't. Also, don't think I'm going to teach you to play piano, I'm not. But the piano may be the clearest vehicle for visualizing musical theory. I would offer to you as a strong suggestion that you obtain a keyboard. They're cheap. It doesn't have to be anything fancy; Yamaha & Casio both make inexpensive models. Hell, even a toy will do, as long as it has more than one octave.
Start with the easiest key; the key of C Major. Why is that the easiest? Because it has 0 sharps or flats.
Plainly speaking, it doesn't use any of the black keys on the piano.
There are 8 notes that make up the scale.
To get from Do to Ra, we go from the C note to the D. If you are looking at the piano keyboard, you note that there is a black key in between them. Because we skip that key, this is called a "whole step". So, from 1 to 2 (Do to Ra) is a whole step. On the guitar, we skip one fret.
To get from Ra to Me, we go from the D note to the E. Again, if you are looking at the piano keyboard, you note that there is a black key in between them. Because we skip it, it is again a whole step.
To get from Me to Fa, we go from the E note to the F. Here, there is NO black key in between them. Because we are moving from one key to the one directly next to it, this is called a "half-step". On the guitar, we finger the next fret.
To finish off the scale;
Fa to So = F to G, skipping the black key, Whole step
So to La = G to A, skipping the black key, Whole step
La to Ti = A to B, skipping the black key, Whole step
Ti to Do = B to C, no black key, half step
So, to recap, the major scale, where W = Whole Step and H = Half Step, the intervals for the Major scale = WWHWWWH
Let's also, now enhance this:
Notice that the 5th note of the Cmajor scale is G. Draw a circle, with C at 12:00. At 5:00, put a G.
G is the second stop on the circle of fifths. The key of G would be the next easiest key, because it has 1#. As a side note...(no pun intended), a black key makes the same sound everytime you press it. Whether you refer to the black key in between C & D as C# or Db depends on what key your are in.
The fact that G has one sharp becomes evident when we apply the same set of intervals (whole steps/half steps) when starting at G instead of C.
2=Ra=A (Whole Step)
3=Me=B (Whole Step)
4=Fa=C (Half Step)
5=So=D (Whole Step)
6=La=E (Whole Step)
7=Ti=F# (Whole Step)
8=Do=G (Half Step)
Now, if we move clockwise again to the 5th of G, our next stop is D. D has 2 sharps.
2=Ra=E (Whole Step)
3=Me=F# (Whole Step)
4=Fa=G (Half Step)
5=So=A (Whole Step)
6=La=B (Whole Step)
7=Ti=C# (Whole Step)
8=Do=D (Half Step)
Next stop, the 5th of D is A. A has 3 sharps.
2=Ra=B (Whole Step)
3=Me=C# (Whole Step)
4=Fa=D (Half Step)
5=So=E (Whole Step)
6=La=F# (Whole Step)
7=Ti=G# (Whole Step)
8=Do=A (Half Step)
And so on, and so on.
Now you know that when you look at sheet music, if you see a song with 2 #'s in the key signature, that it is in the Key of D.
But we said it was a circle right? And so far, we have been working clockwise. We can also move counter-clockwise.
Start again at C. If we move backwards 5 (8-7-6-5-4 [Do-Ti-La-So-Fa]), we end up on F. The key of F has 1 flat (b). And we determine this no differently than the # scales. (On our circle, make a mark at 8:00 and put an F there).
2=Ra=G (Whole Step)
3=Me=A (Whole Step)
4=Fa=Bb (Half Step)
5=So=C (Whole Step)
6=La=D (Whole Step)
7=Ti=E (Whole Step)
8=Do=F (Half Step)
The Fa note of the F scale (or fifth note working backwards) is Bb. The key of Bb has 2 b's.
2=Ra=C (Whole Step)
3=Me=D (Whole Step)
4=Fa=Eb (Half Step)
5=So=F (Whole Step)
6=La=G (Whole Step)
7=Ti=A (Whole Step)
8=Do=Bb (Half Step)
Chew on that for a while, and we'll move on.