R is free to download and use. To do so, simply go to r-project.org and select "download" using the 0-Cloud mirror (or whatever mirror is most appropriate for your area). The default configuration is fine.
You will want to use RStudio with R - it is an interface that makes it easier to work in R, including by making commands consistent across operating systems and much more. The Open Source Edition is free to use and all you should need for getting started.
If you would prefer not to install R and RStudio on your computer, affiliates with WSU also have the option to use this through our High Performance Computing (HPC) system, BeoShock. RStudio is one of the tools available through its OnDemand platform.
There are many packages available to extend R functionality, some built-in upon installation and others accessible from the Comprehensive R Archive Network (CRAN). You will find some linked directly below.