1. Use the Crop Tool to outline the image
2. Put a checkmark in the Perspective box on the toolbar
3. Click on the Checkmark on the toolbar.
4. Recrop if necessary
5. If the image looks out of proportion after you've straighten the side(s) you can use Select/All and then Edit/Transform/Distort to make whatever corrections you need.

There are other distortions in pictures that occur from lens characterisitics, such as barrel distortion which causes bulging sides and pincushion distortion from a tele lens which causes the sides to bend inwardly. You can correct those in CS2 and CS3 by selecting all and then going to Edit/Transform/Warp and dragging the boxes to straighten out the curves.

There are excellent instructional videos for using Photoshop on Russell Brown's website: http://www.russellbrown.com/tips_tech.html