This is different depending on the Word version you have. In 2003 this can be done by changing the font in the normal.dot file. This is used by default as the template each time Word opens a blank document. Here are the steps from Word help.
1.On the File menu, click Open, and then navigate to C:\Documents and Settings\user name\Application Data\Microsoft\Templates.
If no templates are listed in the Open dialog box, click the arrow next to the Files of type box, and then click Document Templates.
2.Double-click the Normal.dot file to open it. To be certain that you're working in the default template, check to see that Normal.dot appears in the Word title bar.
3.Make any changes you want, using the menus and dialog boxes just as you would to change default settings for a document, but remember that any changes you make to Normal.dot will be applied to documents that you create in the future.
4.When you have finished, on the Standard toolbar (toolbar: A bar with buttons and options that you use to carry out commands. To display a toolbar, press ALT and then SHIFT+F10.), click Save .
Word 2007 treats this process differently. You can adjust the template file more directly. Here are steps from the Office website.
1.If your document already contains text that is formatted with the properties that you want to use, select that text.
If you are starting from a blank document, go to step 2.
2.On the Home tab, click the Font Dialog Box Launcher, and then click the Font tab.
3.Select the options that you want to apply to the default font, such as font style and font size. If you selected text in step 1, the properties of the selected text are set in the dialog box.
4.Click Default, and then click Yes.