If you are running an older version of MacOS, you will have to install an older version of R.
Once the download completes, open the installer and follow the directions to install R on your computer. So long as you are running MacOS 10.13 or higher just click the first link for the PKG, which will download the installer for the most current version of R (4.1.1 as of August 16, 2021). This is where the installation directions diverge depending on your OS. These should all be the same, but I tend to choose the Iowa State University link under USA.
To install R, navigate your web browser to: