R help - Oct 2013 - How to establish the order of a time series so as to fit ARIMA

I have sales data for years 2011, 2012 and upto september 2013. I have
applied the decompose function to filter the trend , seasonal and random
component. however i would like to apply the predict function  to forecast
the sales for the rest of year and into 2014. How can i establish the order
of the time series so as to fit ARIMA.

Charles.

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