By default, Word seems to revert to Times New Roman 12 point font.  However, you might wish always to use another font, and instead of having to change font every time you create a new document, it is easier to change the defaults.

To do this, select the Format/Font... menu option.  You will see the following dialog box, which will allow you a great deal of latitude in selecting different font options.

The important thing to remember is to click the ‘Default...' button at the foot of the dialog box before quitting.  This will save the font settings you have chosen into a configuration files called ‘Normal.dot', which Word uses for its default settings whenever it starts up.