Get Ruby on Rails up and running

Install latest version of Xcode from the Mac App Store (4.4+)

Install Homebrew

/usr/bin/ruby -e "$(/usr/bin/curl -fsSL https://raw.github.com/mxcl/homebrew/master/Library/Contributions/install_homebrew.rb)"

Test your configuration with Homebrew (you can safely ignore the warnings about X11 missing)

brew doctor

Install Redis using Homebrew

brew install redis

Enter these 3 lines to automatically start Redis

mkdir -p ~/Library/LaunchAgents
cp /usr/local/Cellar/redis/2.2.12/io.redis.redis-server.plist ~/Library/LaunchAgents/
launchctl load -w ~/Library/LaunchAgents/io.redis.redis-server.plist

.... to continue