I am not a 'fan' of Apple; but I realize many people love them. Installing R on a Mac OS *can* be a bit tricky. Apple updates their OS approximately every year. So, the first thing you will need to do is determine which version of the Mac OS is installed on your machine. 

Then, you will visit the CRAN R for Mac page (https://cran.r-project.org/bin/macosx/) and read it thoroughly to determine which version of R will work with your OS. Then, you can download and install the correct version of R. 

At the very least, you will probably also want to install XQuartz (https://www.xquartz.org/). This software is particularly important if you want to use R packages which utilize a window or GUI (such as Rcmdr). 

While installing XQuartz you will probably be prompted to install Xcode; you will need to install that as well. 

In rare instances you may also have to visit the R for Mac Development Tools and Libraries page (https://cran.r-project.org/bin/macosx/tools/) to download and install clang and gfortran.