Ruby

First off, check to see if you already have Ruby installed. Depending on your operating system, you will either navigate to the terminal (Mac) or command prompt (Windows) and enter this in on the command line.

ruby -v

This should return the version of Ruby that is on your computer. If it doesn't, you need to install Ruby.

Installation

Mac

For Mac users, the easiest way is to use Homebrew. So install that first.

https://www.ruby-lang.org/en/documentation/installation/#homebrew

Windows

I have not used Windows in a long time so can't personally vouch for this but the internet recommends RubyInstaller. It's the whole package.