Question

How can you keep the horizon straight in a photograph?

Answer 1

You can use things like fences and other things with lines in your photograph. You can also use a tripod so that you know for sure that your camera is not tilted at all.

Answer 2

Do you mean to keep the horizon parallel to the horizontal edge of the image?
If that's the case,
1. use a tripod that comes with a level. Those that come with two horizontal levels at 90 degrees to each other are more accurate. There may be level attachments available, check with your photo-dealer.
2. Many modern cameras are equipped with this option. For example, the Nikon 7000. In that case, you don't need a tripod.

If you mean the horizon becomes a curve in the picture, it could be due to
1. the focal length is too short which causes distortion. For example, fish-eye lenses (14 mm or less) gives a more or less circular image. If possible, use a lens with a longer focal length.
2. If the camera is tilted, objects will be distorted (loses parallelism).