Change the font with a blank document open. Then click save as, and save the file as a new template (select "Word Macro Enabled Template, .dotm" under the format drop-down menu in the save as dialogue box) in the user templates directory.
Also, you'll probably want to ditch the double spacing. You can do that as well, but it's already selected as single spacing. You'll have to turn off the spacing before and after new paragraphs. By default, Word:mac 2008 defaults to 10 points after a paragraph. You should set this to zero. (You can find this in the Alignment & Spacing category in the format popup box.)