When designing just about anything in Adobe PhotoShop you will likely find it necessary to crop out certain parts of a layer. There are a couple of different ways to perform this action but not all are considered equal. The obvious choice would be to simply grab your marquee tool, draw a marquee over the area that you want, select the inverse (ctrl shift I) and then hit the delete key. This maybe suitable if you are not concerned about moving around your image later to crop it differently during your editing process, but 95% of the time your are going to want to edit the placement of your images.
Fortunately, in PhotoShop, we have Layer Masks at our disposal. When using a layer mask you can still use your marquee tool to make a selection as you would with the previous method but this time, instead of removing the parts of the image that you no longer want to see, you will need to select the inverse and then simply select the layer mask icon at the bottom of the layers palette.
This will add a layer mask to the current layer that you have selected. The layer mask will do exactly as you would expect by it's name, it will provide a mask to the layer based on your selection, hiding all of the parts of the image that you want to hide. The main benefit to this method is that it is not harmful to the original image and if at anytime during the rest of your design process you want to move the image placement within the crop you can do so. Simply click the link area between the layer icon and the layer mask icon in the layers palette to unlink them. Once they are no longer linked you can select the move tool (v) and move the image around in the crop area. As you do you will notice that you can move the image position anywhere you would like.