"time series forecasting help"

Hi there,

I just downloaded rapid miner and have been playing around with it for a couple of hours.  What I am looking to do is input weekly sales data for the past 3 years (date,units_sold)  and try to build a model, or a series of models to forecast future sales.  What I'd also like to do is see if I can find any seasonality in the data - maybe through an autocorrelation function.  

I'm not sure how to get started and wanted to know if someone could point me in the right direction.

I apologize if this is in the wrong thread.