The crop tool allows you to remove portions of an image to create focus or strengthen your composition.

To change the editable area of your image, choose: Image > Canvas Size.

You can access the brush settings from the Brush palette / panel. Choose: Window > Brushes.

A layer mask is used to hide portions of a layer and reveal portions of whatever layer is below it. You can modify the mask by using the brush tool and painting in either black or white.