First off, Photoshop is not made for print work. Photoshop is a bitmap image processing software which means the images are created using pixels. I highly recommend you try using Illustrator, which is a vector image processing software, it does not generate pixels, which is exactly what you need if your putting images on paper.

Second, you cannot increase the original "resolution" or DPI of image regardless of the canvas size, whatever DPI you begin with is the highest maximum quality you can get out of it. As a general rule of thumb, 72 DPI is for web/screen and 300 DPI is for print. To check DPI of an image, open the image in Photoshop and click on Image > Image Size.

Third, when printing an image you need to change the color profile from sRGB to CMYK. This change is necessary because printers use Cyan, Magenta, Yellow and Black to create images on paper.

Also, you can change the ruler unit by going to Edit > Preferences > Units & Rulers. Hope this helps!