If you're new to Lisp, http://ycombinator.com/arc/tut.txt isn't a bad start. But there are countless Lisp tutorials out there.
As for online documentation: http://files.arcfn.com/doc/ + http://www.arcfn.com/2009/06/whats-new-in-arc3.html; see also http://arclanguage.org/item?id=12228, wherein I technically have another suggestion for learning Arc without the web ;).