Discover the Crop tool for Image Editing

Grab the crop tool and just drag it across the image so you have a general marquee selection around the area that you'd like to keep.

You'll notice how it darkens everything else? This is because that is the area that you're going to get rid of.

Adjust the corner handles (hold down shift to retain proportion for the corner handles) to cover the area that you'd like to keep.

Then press enter. The whole document now becomes this new size (remember the Info palette that shows the measurements of the bounding box? Now's a great time to use it if you need an exact cut).

See how much more interesting the photo is once you get rid of all that extra space (even a lot of MY photos could use a little cropping)?! It focuses the whole image nicely on the hired for photography family.

Keep in mind that once you do the crop, this is the new file size unless you go back in the history palette to redo it. You can always Save As and name it something else to retain a copy of the original still in your hard drive).

Or you can Save As and save it in a different format with the same name to save a copy and retain the original pre-cropped.  If you know you're going to keep the cropped version as the master then just Save (override the previous version of the file) to retain your new state of the document.