If you are then you'll need to start here. Basically Microsoft has provided these as "Guidelines" to start planning for your DM implementation with MOSS. I have collated the below links to some useful resources that I have used in doing workshops for a few clients around starting to understand and implement DM with MOSS.

From TechNet:

Plan Document Management - A very good starting point from an implementation planning point of view. There are some worksheets that may come in handy.

If you are looking at a full MOSS deployment then here is the link to get you started. (Lots of reading!)

Planning and Architecture for Office SharePoint Server 2007 - Comprehensive planning for all that MOSS has to offer.

From Bob Mixon:

MOSS 2007 Document Management Services - Document Centralization Part 1 - Very useful article. Bob is a fellow MVP and has some valuable insight on his posts on planning for DM.

MOSS 2007 Document Management Services - Document Centralization Part 2 - Second article in the series.