In order to install Jekyll on your Mac you need to install a couple of things first. One is Homebrew, Ruby and then Jekyll.
If this sounds like Spanish don't worry its not.
Go to the App Store and download Xcode and Install.
It may ask you to install additional components. Install these also. Once this has been installed. Close Xcode.
Its a black icon like this.
When you open terminal it looks this this. TOTALLY MATRIX MAN.
(/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)")
Some crazy Matrix type stuff should begin to happen. Don't worry this is completely normal. You might need to hit enter to continue or input your password.
brew install ruby
sudo gem install jekyll bundler
Hooray! Installation should be finished. To check you can run the command
jekyll-v
This will show which version of Jekyll you are running. Should be Jekyll 3.5.2