Using the Crop Tool To Straighten a Crooked Photo

Step 1 : I have opened this photo in Photoshop. Notice that it is a bit crooked. We will use the Crop Tool to fix it.

Step 2 : Click the Crop Tool. See the Black arrow. That is the Crop Tool.

Step 3 : When you click the Crop Tool, you must take note of some other settings. See the three red arrows at the top. Make sure that the Perspective Box is unchecked. Set the Shield color. By default it will be set to Black. Click on it. This will bring up the Color Picker. Choose a color other than Black. I have chosen Blue. Now lower the Opacity (the middle arrow).
Also while dragging the Crop Tool across the image without letting go of the mouse button press the Space bar on the key board. This will allow you to move the selection while resizing it.

Step 4 : You have the crop selection. If you move the cursor just outside the selection you will notice the arrow changes to a small double headed arrow. Now just rotate the selection. See above.

Step 5 : After rotating the selection, click the Tick Mark on top. See the red arrow.

Here is the cropped image. It is no longer crooked. That's all. Try it out.

See here also for another tutorial on Crop Tool.