Forcasting, Predicting Future sales figures.

I was just wondering if anyone new or could point me in the right direction, I want to know what the best function or formula for predicting future revenue, I want to be able to make an educated guess at what the month after December revenue would be. I have attached an example doc, with spend for the past 2 years and I want to be able to find out what the next month of data would be.

Thanks