C at the top of the circle has no sharps and no flats. Circle right to see which key has how many sharps, and circle counterclockwise to see which key has how many flats. Notice the addition of one accidental for each step taken. The key signature notation next to the letter shows where on the staff these accidentals are placed. Each upper case letter on the outside of the circle corresponds to the key signature, and the lower case, inner circle, letter connected to it denotes that key's relative minor, which of course is written in the same key. It's certainly less complex than this explanation would indicate. You'll notice that as you move clockwise from C to shining C you are traveling in fifths, i.e., G is a fifth to C, D a fifth to G, etc.; and as you travel counterclockwise you circle down in fifths. Even as you continue passed the sharps and into the flats you still travel in fifths ~F# being a fifth to B, and so forth. You'll also remember that you add one accidental for each step in either direction.
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01 ~ Music ~ Circle Of Fifths (wOut Relative Minor)
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02 ~ Music ~ Circle Of Fifths (WITH Relative Minor)
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