This is how I typically do it:
1. Open the image.
2. Type F to go to the view where you can have some room to play with the image.
3. Select the entire image with the marquee tool or by typing CTRL+A.
4. Go to EDIT, then TRANSFORM, then ROTATE. You will see the box appear around the image.
5. Using the mouse, place the cursor near one of the corners and rotate the image until it is as straight as you'd prefer.
6. At this point, you'll notice that the corners of the image will have slivers of triangles of whatever the background color is. To eliminate that, zoom in to one of the corners and select the part of the image that does not include those slivers.
7. Go to IMAGE, then CROP.
8. (Optional) If you'd like to make the image the same size as it was, go to IMAGE, then IMAGE SIZE, and resize.