See http://word.mvps.org/faqs/customization/CustomizeNormalTemplate.htm.

The default document template is normal.dot. You can make modest changes to
this document, but I would urge you not to do any more than that as it can
have knock-on effects to other documents.

If you want to make radical changes to the default template then instead
create your own template and drop a link to it on the desktop. As the
default action for templates is New, clicking the link will open Word with a
new document based on your template.

For new documents while Word is open, add a button to the toolbar to run a
macro containing the line
Documents.Add Template:="C:\template path\Template name.dot"