My Mac has Ruby on it, do I still need to install Ruby/RVM?

Yes, yes, and… YES! Mac comes pre-installed with a non-feature complete version of Ruby. While it may work, it is not maintained to the same standard. 

You should definitely install rvm (or another version manager of your choice), and then update Ruby to make sure that, while you computer may have a pre-installed version, you will be safe with the most up-to-date stable version around!