You don't have to know how to calculate the correlation coefficient.
All you need to know is really how to use it to compare different sets of data.
If r = 1, then all data points are in a straight line.
If r > 0 then the correlation is direct [when x increases, y increases].
If r < 0 then the correlation is indirect [when x increases, y decreaes].
The larger the value of r, the more correlated the data points are.
That's pretty much it I think.