Most of Word’s formatting dialogs include a Set as Default button that lets you create a default setting for fonts, paragraph format, page layout, and much more. Click on that button, and Word offers you a choice of making your chosen format the default for the current document or all documents based on the current template, which will almost always be Normal.dotm.
But you can’t modify everything from these dialogs. I want my documents to have no page number on the first page, but a page number in the upper right on later pages. To do this, navigate in Windows Explorer to the folder that contains the default Normal.dotm template, which is in C:\users\\AppData\Roaming\Microsoft\Templates; you may need to set Windows’ View options to show hidden files and folders so that Windows will show you the folder. (On a Mac, use Spotlight to search for normal.dotm; highlight the filename in the search hits, and press Cmd-R to open the Templates folder. In recent Word versions, its address in your user folder is Library/Group Containers/UBF8T346G9.Office/User Content/Templates) Double-click on Normal.dotm to open it in Word.
Word will create an empty file with no name. Press Return a few times to create empty paragraphs, then Ctrl-Enter to insert a page break, then press Return a few more times to create new empty paragraphs. Back on the first page, use the Insert tab to add a header or footer. In the Header and Footer tab that appears in the Ribbon, choose among options to use a different first-page header and similar choices. Click in the document, go to the second page, click in the header or footer, and insert page numbering or use any other option. When you’re done, click in the document, press Ctrl-A to select the entire document, and delete the selection. Word will clear out the empty paragraphs, but will keep your instructions for headers and footers.
(Credit: Microsoft)
Now choose File >Save As, and very carefully navigate to the same Templates folder where you opened the template. Be careful when navigating, or Word will take you to some other folder. When you’re in the Templates folder, choose Word Macro-Enabled Template (*.dotm) as the file format, and save the file with the name Normal.dotm, replacing the existing file. Then close down Word.
The next time you open Word and create a new document, the settings you chose for headers and footers will automatically apply.