You'll find the crop panel on the right in the vertical toolbar. You can also use the keyboard shortcut 'c' to quickly open or close the crop panel. From here you can crop, resize or level your images.
Hit 'Done' or close the panel with 'c' to apply your crop. Hit 'Reset' to remove the crop and revert back to your images originally shot aspect.
As with your ratings, the crop applied to each image will be stored in the associated XMP file in your source folder.