As a first time user of Ruby and Rails, I was able to setup a sane environment on Ubuntu Server 12.04 (x64) in less than 10 commands:

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cd $HOME
curl -L https://get.rvm.io | bash
source $HOME/.rvm/scripts/rvm
rvm requirements
rvm install ruby
rvm use ruby --default
rvm rubygems current
gem install rails

And, if you already have a rails app to run, continue along with:

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cd myapp
bundle install
rails server

This will get you up and running with a dev server.