For those looking for a straightforward introduction to forecasting, see the free e-book, Forecasting: Principles and Practice by Hyndman and Athanasopoulos: https://otexts.com/fpp2/
That's an excellent resource that I've used heavily (along with their awesome forecast package for R). I also found QuantStart's series on Time Series Analysis (scroll down a bit: https://www.quantstart.com/articles) quite helpful, and it gets a little deeper into the math so I had a better understanding of why certain things are done.