- Open your image in photoshop
- Select the measue tool
- Hold down your mouse button and draw a line along the horizon
- Select Image> Rotate Canvas> Arbitrary
- Click ok on the rotate canvas box
- Select the crop tool
- And draw a box for the area you want to crop
- Double click in the box or click the tick(top right) or hit enter(return key)
- You now have straightened your image.
- Save your image and continue any work on it.