I'm sure there's a simple way to achieve this but I haven't found it. When I create a Shape Layer and try to add a Mask to it by drawing another shape with the Rectangle Tool it applies it as another shape but if I use that same technique for an image layer it applies it as a Mask.
I've been getting around this by creating a New Solid and applying a mask to that and then copying and pasting onto the Shape Layer and then deleting the New Solid... there has to be a better way.
Along the top of the window, next to the tools, while you have a shape tool selected, there are two buttons: one with a star, one with a sort of dotted-line-shape thing. The star will make the tool create a shape, while the DLS thing will make the tool create a mask.