My camera makes straight lines crooked in pictures?

I have a nikon and when I take a picture of for example, a corner of a wall, it would make the straight line of the corner crooked and black lines would be on them too. What am I doing wrong? Or is it the camera?

You probably do not have the cameras back parallel to the wall. And are using a wide angle lens (18mm on your lens is a wide angle lens)

Make sure the the camera is 1/2 the distance to the ceiling and its back parallel to assure images like this one.

Got an example you can link?