There are several ways to add a mask to an image. You can create a new mask layer completely showing or hiding the underlying layers. You can also create a mask from an image, or a selection.
Note: Upon selecting the background layer to create a new mask, you are prompted to promote the background to a full layer. When the background layer is selected when you create a new mask, you are prompted to promote the background to a full layer.
Ways to create masks:
Creating Masks from Selections