How to install stable ruby 1.9.3 on rails 3.2.13 on Ubuntu 12.04 LTS with RVM

1. Install rvm /version controller/

curl -L https://get.rvm.io | bash -s stable --ruby

2. run

source ~/.rvm/scripts/rvm

echo "~/.rvm/scripts/rvm" >> .bashrc

3. run

rvm requirements 

4. run

rvm install ruby-1.9.3-p484 

rvm use 1.9.3 --default

5. run

sudo gem install rails -v 3.2.13

sudo gem install bundler

bundle install 

6. gem install sqlite3-ruby 

Also for reference, you can see http://brightbox.com/docs/ruby/ubuntu/ 

Here has great instruction how to upgrade ruby by sudo apt-get install bla bla bla