The Circle of Fifths is a circle process that is based on the dominant note starting and ending at C. It tells you the order of sharps and flats, i.e. C has no sharps or flats, G has 1 sharp, etc.
If in doubt in an exam it is the third note in a standard major triad. E.g CEG is a c major triad and the dominant is G
This is the circle C-G-D-A-E-B-F#-Db(C#)-Ab-Eb-Bb-F... and back to C
There is also a circle for the relative minors - starting and ending at A (no sharps or flats)