The rbenv wiki provides a great guide to install ruby 2.0.0-p0 https://github.com/sstephenson/ruby-build/wiki#installing-200-p0-on-osx-107 which covers many of the issues, yet it wasn't enough for my setup.
First of all,
brew update failed miserably. My local brew repo was a mess. When
brew update fails a lot of unstaged changes appear on the directory. So
git clean -f gets rid of that, but you should also remove any uncommitted files doing some
rm this; rm that.
Then, if you get "Error: The /usr/local directory is not writable." you might want to take a look at your write permissions:
ls -l /usr/ throws a 755 mode with admin as owner and wheel as group.
Changing the user group to admin, worked for me, since my user is on that group.
sudo chgrp admin local
After that, everything worked,
brew update and all of the steps explained in the rbenv wiki.
To get to your brew directory type:
cd $(brew --prefix)