Though the traditional way of cropping a photo is with the crop or marquee tools, there is a quicker way which works especially well if you have a solid color background or one with very little detail.
Look at Image>Trim. It will give you the sampling option, either upper left or lower right corners I believe. It will automatically crop your image to the main subject, if it is well defined against the background. This comes in handy when Batching large numbers of similar photos. I set this up as an Action in my Actions Palette when I was working on a sporting goods website this past week as an easy way to clean up unwanted background space.