You want the camera to rotate on its Y axis for 20 degrees? Keyframe it.

The camera is just like any other object in the scene. I assume you're using the "pers" camera. It's the default camera that lets you move all around the scene. Anyway, choose it in the Outliner then set up your start and end keyframe.

Just so you know, I think most people would agree that it is better to create a new camera to use to shoot the scene. The default cameras are tools to let you see the scene while you're building and animating it.