Change the Normal template (Normal.dotm ) - Applies to: Microsoft Office Word 2007
quote:
Any changes that you make to Normal.dotm will be applied to documents that you create in the future.
Note: If Normal.dotm is renamed, damaged, or moved, Word automatically creates a new version (which uses the original default settings) the next time that you start Word. The new version will not include any of the customizations that you made to the version that you renamed or moved.
For exact How-to click the link.
Assuming you want to do that for Word...if anything else (for Office)
Microsoft Office Online - Help and How_To is a good place to start your search