Most photographers use a size of 18 X 12 to compose photographs. DSLR cameras mostly shoot for this format also, with a few exceptions. The basic rule of thirds indicates that the focal point of the photo should be one third in from the side edge, and one third in from the upper or lower edge of the photo. In the case of a human subject, the focal point of the photo is usually their leading eye. Ideally, this rule works very well for most photos, but it's not cast in stone.
The main focal points for a photo can be found in the image above. Draw a line from the upper corner to the lower diagonal corner to bisect the image. Now from each OPPOSITE corner, drop a line that meets the original line perpendiculy. These are the "ideal" natural focal points of an image. Strangely enough, they comply with the rule of thirds.
Isn't this photography stuff simple???