Textbook
Forecasting: Principles and Practice, Hyndman & Athanasopoulos (3rd ed., 2020)
Online course
Datacamp: Forecasting Using R by Professor Rob J Hyndman
R and RStudio
The first thing you need to do to get started with R is to install it on your computer. R works on pretty much every platform available, including the widely available Windows, Mac OS X, and Linux systems. If you want to watch a step-by-step tutorial on how to install R for Mac or Windows, you can watch these videos:
Next you can install Rstudio. Remember, you must have R already installed before installing Rstudio. If you want to watch a step-by-step watch the vedio here