The first step in using the crop tool to alter perspective is selecting the crop tool. You can get the crop tool by clicking on it in the tool palette, or by pressing C.
Here is my starting image. I would like to crop down to the window while getting rid of the "falling away" look.
First I use the crop tool and make a rectangular crop around the window.
Next I slide the sides of the crop in so they just touch the widest part of the window. This will make it easier to adjust the perspective later.
I go up to the option bar and click on Perspective.
With perspective checked I can grab the top corner boxes of the crop boundary and while holding down the Shift key, slide them in. I slide them in so that the edges of the crop area line up with the edges of the window as shown here.
Once I have the amount of perspective correction set I can slide the edges of the crop area around until I find what I want. When I'm happy, I press Enter
And here is the window, with the perspective corrected!