I know this circle of fifth thing can be confusing, but have you ever seen the visual? Here's a link. Look at this while you read my explanation below, and I'm sure you'll get it.
http://lessons4violin.com/yahoo_site_admin/assets/images/fifths_pic.17692450.gif
Keep in mind that the circle of fifths is nothing but a tool to help you understand how scales and keys are formed, as well as how chords relate to each other. Something that's much easier for a lot of people to do by ear. But.... I find the combination of the two as the best solution.
So think of it this way
In the middle you have C (at 12 o clock) = no sharps, no flats
Go clockwise and you have all the "sharp" keys: G, D, A, E, F# (F# is at 6 o clock)
G has one sharp
D has two sharps
A has three sharps
E has four sharps
F# has five
Next question is: What sharps?
[U]F is always the first note you "sharp"[/U], so in G (where there is only one sharp) you sharp F, so that you have F#!
Next note you sharp is C, so in the key of D (where there are two sharps) you have the notes F# and C#.
Next note to be "sharped" is G, so in the key of A you have F#, C# and G#.
This way you continue clockwise, by first determining how many sharps you have by counting (again: c = 0, g=1, d=2 etc.) and then determining what notes to sharp by always starting with F, and then going clockwise in the circle of fifths (F, C, G, D, A)
Now.... On the left side of the circle of fifths you have the flat keys. Going counter clock wise you have
F, Bb, Eb, Ab, Db, Gb (Gb is at 6 o clock... Gb is the same note as F#)
Same procedure again: How many flats? Remember we're going counter clock wise:
Bb has 1 flat
Eb has 2
Ab has 3
etc.
What notes do you flat? Again: We're going counter clock wise!
[U]B is always the first note you flat[/U]
E is the next one
A
etc.
So let's take a couple of examples:
Bb has two flats
The first note is always B, and after that is E, so the Bb scale is
Bb, C, D, Eb, F, G, A
Db has five flats
B, E, A, D, G
So the Db scale is
Db, Eb, F, Gb, Ab, Bb, C
Now, the relative minor is the minor scale that has the same amount of sharps or flats as a given major key.
You can find the relative minors in the center of the circle. Fx Am has no sharps- just like C!
Bm has two sharps, just like D
Gm has one flat, just like Bb!
So that's how you use it to construct scales! Hope this helped. If not, don't hesitate to let me know!
Anders
PS: To answer your question, all the sharp keys could potentially be flat keys, but since you don't want more accidentals than necessary, you switch when you reach "6 o clock" (= Gb/F#)
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