I'll be the dick that suggests not using org mode for class notes; I typed up my lecture notes in it 4 years ago (for the class I teach) and there have been several backwards incompatible changes to org mode that required a few hours of debugging to get indenting, etc. to work correctly with my old files (yes, in a plain text outline).

Now, these are huge files, so my use case is pushing it, but you'll want to be able to use and modify your lecture notes for years, so I'd suggest something lighter weight like a plain markdown file.

Whatever you do, take care of version control and backup.

The easiest thing to do is to install a variant of emacs on your machine (aquamacs is fine but stuck at version 23 of emacs). It probably comes with org already at this point.

I assume the things you'll want to track are facts and tasks. You'll want to capture as quickly as possible for later organization.

It really is as simple as typing to get started. Then you can go back later and re-organize your notes, tasks, etc. according to whichever system you prefer.

Some tutorials that seem to be well-paced for org newbies are available at http://orgmode.org/worg/org-tutorials/.

http://sachachua.com/blog/2008/01/outlining-your-notes-with-... (old, but you probably want the basics to get hooked first :-))

http://doc.norang.ca/org-mode.html (includes a good guide to the state of tasks, among lots of other things to orgify just about every aspect of your existence)

Just download Emacs from here: http://emacsformacosx.com

Org-mode is perfect for taking notes in class, especially if it's a programming class. In that case, you could use Org-Babel[1].

[1]: http://orgmode.org/worg/org-contrib/babel/