Use this command and follow the instructions:
curl -L get.rvm.io | bash -s stable --rails
Other options include(any gems imply --ruby):
--ruby=1.9.3
--gems=rails,gist
Optionally on OSX install homebrew, use brew
command to install whatever database engine you need, ie brew install sqlite
/usr/bin/ruby -e "$(/usr/bin/curl -fksSL https://raw.github.com/mxcl/homebrew/master/Library/Contributions/install_homebrew.rb)"
Finally, start working with rails:
rails new myproject