Forecasting is needed for effective and efficient planning in business areas like inventory stocking and retail POS staffing. If you wish to have a comprehensive introduction to forecasting methods then you will find a free online textbook on this topic quite useful.
“Forecasting: Principles and Practice” is written by Rob J Hyndman and George Athanasopoulos from Monash University, Australia. It uses R to help readers learn how to forecast using R. R is an open source software, which is available for free for every operating system. It is a very useful tool not only for forecasting but for all statistical analysis.
The free online book is available at https://otexts.com/fpp2.