Sales Forecast with recursive loop

Hello Dear Forum,

i need your help and I hope you can understand my question.

I have sales data from 2013 - June 2015 and should make a forecast in sales for July 2015.

Now I thought about to do this best with a recursive loop but unfortunately I’m lost to this one.

I hope somebody can help me to create a easy and understandable workflow for my problem.

Many thanks in advance!