Notice the squares on the corners and sides of the selection marquee. These are called handles because you can grab onto them to manipulate the selection. Move your cursor over each handle and you'll notice it changes to a double pointing arrow to indicate that you can resize the crop border. Make some adjustments to your crop selection now using the handles. You'll notice if you drag a corner handle you can adjust the width and height at the same time. If you hold the shift key down while dragging a corner handle it constrains the height and width proportions.
You'll find if you try to move the selection border to just a few pixels from any of the document edges, the border automatically snaps to the document edge. This makes it difficult to trim just a few pixels from an image, but you can disable snapping by holding down the Ctrl key (Command on Mac) when you get near an edge. You can toggle snapping on and off by pressing Shift-Ctrl-; (Shift-Command-; on Macintosh) or from the menu View > Snap To > Document Bounds.
