You can't with Outlook 2003. Printing selected pages wasn't enabled until SP2 of Office 2007.
If you really need to just print a single page I'd recommend either printing to PDF then printing only the 1st page of the PDF; or copy/paste the message to Word and then print only the first page from there.
Outlook Q&A:
http://www.officeforlawyers.com/outl...tlookqanda.htm