This one is so simple it’s crazy to write a post about it. At this time, go to http://nodejs.org/ and download the installer and install that way.
Extra credit: There is one little extra tool you will may want and it’s called Homebrew. This is the thing that makes installing packages for OSX so super simple. You will be doing a lot of “npm” with Node. That is how you install packages. Well, OSX can have this same yummy greatness with Homebrew.