Let's say there is only a part of an image you want to keep or work on. In this case you will want to crop off the other part of the image. To do this, perform the following steps:
Select the tool of your choice: Rectangle, Lasso, or Ellipse.
Use the selected tool to draw your selection.
In the menu bar, select Image | Crop to Selection or use the keyboard shortcut Ctrl + Shift + X.
If this doesn't work, go to Image | Crop to Selection. If it is grayed out, make sure that you have something selected.