Are you using Photoshop the full version or Photoshop Elements? Elements is currently on version 6.0 and it's a little different from the full version of Photoshop.
Basically the easiest way is to use the crop tool which should be on the side menu bar. You can see across the top bar changes to the size you want for your image - select your size from the drop down or choose the "no restrictions" option. (These screenshots are from Elements 5.0, but it should be very similar in 6.0)
Then you just drag the crop tool across the image. What you don't want in the image will be grayed out. Then click the little green checkmark and ... voila! ... your crop is done.
Hope that helps.
.