Install tree
brew install tree
Install rbenv and a recent stable version of Ruby
brew install rbenv
rbenv install 2.3.3
Install [rspec] because for some reason installing bundler doesn't do this.
gem install rspec
Install bundler
gem install bundler
Read about how to use Bundler.
(To create sample project "puppy"):
bundle gem puppy
Learn Ruby from scratch: https://www.ruby-lang.org/en/documentation/quickstart/
Further Ruby study: https://www.ruby-lang.org/en/documentation/
Ruby Koans: http://rubykoans.com/