Correct perspective distortion

Step 1 - Open the photo
Open the photo you wish to correct.
Step 2 - Select the tool
From the Tools toolbar, select the Perspective Correction tool-- it's grouped with the Deform tool.
Perform this step for me.
Step 3 - Mark the lines that need correction
Notice the correction box on the image. Drag the corners of the box over the perspective you want to correct. For example, to straighten a seemingly tilted building, drag the corners to the outline of a tilted side on the building.
Step 4 - Apply the Correction
Click Apply in the Tool Options palette, or double-click inside the image. The marked lines will straighten to a rectangle.
Notes and Tips:
To crop to the largest rectangle with sides parallel to the image borders, mark the Crop checkbox in the Tool Options palette.

Learn More:
View a movie about this topic
Straighten a crooked photo
Rotate an photo