Once upon a time when I still use windows laptop as my workstation, I used to know there is a Ruby One click Installer for Windows. Just notice, another one click installer for mac is born. Visit http://rubyosx.rubyforge.org/ to download.
The good things are this package help to install basic framework to get you started with Ruby on Rails, It done the installation in UNIX way, which installed everything on /usr/local and it replaces the broken Readline library, updates to a current version of SQLite3 and prepares your OSX for Rails, which needs at least Ruby 1.8.4 to run.
After, do not forget to set your environment for /usr/local path.
export PATH="/usr/local/bin:/usr/local/sbin:$PATH"
But if you still prefer the Terminal way to install Ruby on Rails framework on OSX, I have fews good links below: (Just stick to these links, you will be fine)