You can turn auditing several different ways, if you have SharePoint
Server in my opinion I would audit at the document level. This would
require using content types and place auditing on them. This will give
you the freedom to place different
auditing on different types of documents. You set auditing when you
create the content type.
This is done from Site Actions... Site Settings... Under Galleries click Site Content Types...
Once here click create...
Name your content type
Set your content types parent, in this clase in the "select parent content type from" drop down click "document content types"
In the parent content type box for this example select document, select a group if you want... click OK
If you have now you should see a link under settings which are for Information Management Policy settings... click this link.
Now you will see a check box for Auditing... select this and the page
will reload with you auditing settings. You would make use of this by
now using the content type in a doc lib or which ever type you want.
Another way of setting auditing is to set it at a site collection.
You need to be on the root of your site collection to be able to do this, click on Site Actions... Site settings...
Under Site COllection Administration click Site COllection Audit
settings. This will set auditing for the entire site collection.
Good luck,