The crop tool is the most used feature of any graphics program and
commonly used in Photoshop.
By cropping an image you are reducing the pixel size and the overall
maximum print size of the file. Don't drastically crop a photo if
you wish to make larger prints from it and always work on a copy
of the original file.
Simply select the tool from the tool bar and then drag out a crop
marquee on the photo and then double click to crop. It is useful
for consistency if you maintain the original aspect ratio. Drag
the crop marquee and the whole photo, hold down the shift key and
then drag the marquee thus holding the exact shape.