How do I properly re-size an image for submission to Threadless?

Hi there, I am having some difficult understanding how to get my image to fulfill the submission guidelines. The image needs to be high resolution and yet they have specified a file size of no more than 845 pixels wide, 445 pixels tall, and 72 dpi. When I create a new image in photoshop and then drop my design in there I have to use the re-size tool to make it small enough to fit in and it looks ‘pixely’ and crap. I have no idea how to set the dpi at all. If someone could help technologically retarded me out with this I would be most greatful.