RVM (Ruby Version Manager) handles the installation and management of multiple Ruby environments. Ruby 1.9.3 is required for Octopress and using RVM you can install it with ease.
Run the following command from your terminal.
Be sure to follow any subsequent instructions as guided by the installation process.
Install Ruby 1.9.3
Next install Ruby 1.9.3 and you'll be all set.
ruby --version to be sure you're using Ruby 1.9.3. If you're having trouble, seek help here.