The Circle of Fifths is a great tool for the musician trying to write new music or trying to determine what chords or notes are used in an existing or new song.
Western music is based on a specific relationship between notes. Certain distances between a certain set of notes in a chord or series sound better or at least different than other sets of notes with other relational distances. The Circle Of Fifths is a chart showing how these notes relate to each other and is laid out in a manner that complimentary notes are next to each other.
Links below will provide more detail on how the Circle Of Fifths actually works, I just want to make you aware of this musical protractor and hopefully it will help you as well.
First link is an interactive Circle Of Fifths:
Following links are explanatory of the tool: