The Org Mode included in Emacs 24 is version 7. Version 8 has lots of new configuration variables and the exporting mechanism has been rewritten. However, it needs to be installed in an Emacs that has not yet loaded any Org code or files. Here's how you can upgrade your Org:

1. Start Emacs with emacs -q. This skips your personal configuration.
2. You will need an Internet connection for this step. Type M-x package-install, and type in org. This will install the latest version of Org from the built-in package repository.
3. Edit your ~/.emacs.d/init.el (or ~/.emacs, if you're using that instead). Add the following code to the beginning of the file:

   (package-initialize)
   (setq package-enable-at-startup nil)
   

   This will load the installed packages when you start Emacs, overriding the buit-in Org 7 with the Org 8 version that you installed.

   Advanced note: If you've downloaded Emacs Lisp code that should override code already installed through packages, you need to change this to (package-initialize nil) instead, and add (package-initialize t) after your load-path settings.

4. Check your configuration for references to the older version of Org. In particular, look for any configuration related to exporting (ex: (require 'org-html)). You can change those lines to their Org 8 equivalents (ex: (require 'ox-html)), but it's probably easier to just comment them out for now. You can comment out lines by adding ; to the beginning.
5. Save your init.el and restart Emacs (this time, without the -q option). M-x org-version should now start with Org-mode version 8.
6. Review your Emacs configuration for any changes that you will need to make. You can ask the Org Mode mailing list for help if you get stuck.

Good luck!