...open an image, press the 'C' key, drag an area to crop, adjust area with the handles if required, then double click the cropped area to commit the cropping function...
...you can constrain the crop area by dragging a handle and using a combination of 'shift', 'alt/option' or both keys at the same time...
...if not happy with it, crop it further or press command/apple 'Z' to undo and start again...
...choose 'file > save as' to save a copy so you have the original image stored for further use if required...
TIP: To extend the canvas area with a background color to create a border you can drag the crop area outside the image area into the grey surround, double click the crop area and you will have a border based upon the background color in the tool bar (white is the default).
If you want the border transparent instead, duplicate the background layer (command/apple 'J'), delete the background layer, drag out the crop area into the surround and double click to commit the function.
Andrew