Autocorrelation in Excel

I’m no quant wiz but recall enough from my studies from L1 and L2. Essentially I have a monthly time series illustrating the change in subscribers (i work in the telecom industry). When graphing the series I notice that there is some sort of pattern (seasonality or something); I want to know if there is any serial correlation/autocorrelation. I figure if i lag the data for 12 months against itself (i think thats k=12) there will be some evidence to buttress my hypothesis. Any idea how to do this on excel or do i need a stat program? Is my logic even correct…is autocorrelation with the lags right for this task?

let’s say you have data in B2:b500 -> calculate correl(B2:b488,b14:b500) to calculate autocorrelation with lag 12.

coo thanks