Doug,
Not quite sure if you're talking about "creating a new document" or changing the ruler units for an existing document that has already been created.
so...
for a new document, all you need to do is "click" on the drop down menus on the right hand side, opposite the 'width' and 'height' windows in the "New Document" dialog box.
alternatively, with your existing document open, choose "Photoshop" from the top menu bar (to the right of the apple) then "Preferences" and onto "Units & Rulers". Change the units to suit.
Hope that helps you in your dilema.
Should you need some visual aid, I can screengrab the various items.
Cheers,
Gaz