Installing Rails on a fresh system

1. Install Ruby from the Ubuntu repository: sudo apt-get install ruby ruby-dev \
   ruby is the meta package. If you want to explicitly install 1.8 or 1.9, install ruby1.8 or ruby1.9 instead (the same applies for ruby-dev).
2. Do not install RubyGems from the repository but install the version from the webpage instead.
3. Get Bundler: sudo gem install bundler

Rails and other gems for a project should now be installed via bundle install from the project directory. If you need Rails and don't have a Gemfile you can do it old-school: sudo gem install rails -v=2.3.12.

You will also need Ubuntu development packages for MySQL, RMagick, etc. The bundle install call will otherwise fail for such projects.