Don't believe you can in WordPad. It is not an option that can be
defaulted. The default page orientation is set in WP from the default
printer settings. If you set the default printer to Landscape, the new WP
document will start in Landscape.
In the printer folder in the control panel, change the default setting to
landscape. Keep in mind when you change the printing preferences for WordPad in
the control panel it will change for all your programs.