Coke for your data.

Plot the data on a scatter or line chart.Does there appear to be a seasonal pattern to the data?

Do a simple regression without seasonality.

Do a regression calculating the seasonal factors using the dummy variable method.(Remember you need only use three dummy variables as the fourth is implicit in the data itself.)

Which model seems to give the better fit.

 Year2005 2006 2007 2008 2009 2010 Q1 Sales5206 5226 6103 7379 7169 7525 Q2 Sales6310 6476 7733 9046 8267 8674 Q3 Sales6037 6454 7690 8393 8044 8426 Q4 Sales5551 5932 7331 7126 7510 10492