That all depends on what programs you have to do it with, if you have no photo editing programs, the easy way is MS Paint. Open Paint, load the photo, use the ouline tool (dotted rectangle) and outline the entire image, then drag one of the corners to make the image smaller. After that, go into Image -> Attributes, and make the canvas size smaller.
Most basic way to do it, but I would recommend something like ACD See, or Photoshop...