There are probably several ways to go about setting up the page size and cell size but the screen capture should show how things should look.
12.25 X 18.25 is not a normal aspect ratio of a capture from a digital camera so your first step would be to a apply a crop at that aspect ration in the develop module prior to proceeding to the print module.
Also this tutorial should help. Lots of free stuff on Adobe TV.
To follow on with the answer-
You can type the exact cell size you want- just double-click on a cell dimension and type- 12.25 and 18.25
If your image is not in those exact proportions (aspect ratio) then "Zoom to Fit" and "Zoom to Fill" will have different results.
In my screen clip below I have typed the exact dimensions wanted, but note that I had "Zoom to Fit" active so my image shows its print dimensions slightly different.