1.Check your ruby version

ruby -v

2.Install rvm

bash -s stable < <(curl -s https://raw.github.com/wayneeseguin/rvm/master/binscripts/rvm-installer)

3.Install homedrew

/usr/bin/ruby -e "$(/usr/bin/curl -fksSL https://raw.github.com/mxcl/homebrew/master/Library/Contributions/install_homebrew.rb)"

4.Reload your shell environment:

source ~/.bash_profile

5.Find the requirements (follow the instructions):

rvm requirements

6.Verify RVM

type rvm | head -n1

7.install autoconf

brew install automake

8.Install ruby:

rvm install 1.9.3 (or rvm install 1.9.3  --with-gcc=clang)

9.Verify ruby version

ruby -v