Sometimes, your composition rule might be accidentally placing your subject dead-center or you couldn't crop in camera to produce a better composition! In this case Photoshop has the solution for you; you can use it's crop tool to enlarge the canvas area and then use your Content-Aware Fill to add to it.
First thing first and to add transparency around the photo instead of filling the added space with the background color, convert the Background into a layer by selecting Layer > New > Layer From Background (or by clicking on the lock icon to the right of the word Background in the Layers panel).
Place a crop on the area you want to extend and drag the handles outside of the photo area then release your mouse when you are happy with the composition. Apply the Crop so that the area outside the photo is added to the canvas.
Now select any marquee tool to select the transparent areas but do this bit by bit. For example, select the left side of the photo and then apply Fill > Content-Aware Fill.
When done you can select the brush tool and apply 70% opacity to adjust any variations in tones. The end result looks like this...
The photo above was easy to achieve as the background was seamless white. If you have a busier background it is always better to properly compose in camera instead of spending hours in Photoshop removing distractions.