Creating a white smile is easier than you think and this tutorial will show you how:
Once your image is open, zoom in to the mouth area using the magnifying tool or by pressing Ctrl +.
Select the Quick Selection tool and go along the teeth, clicking until they're all selected. On the top tool bar you can change the size of the brush and if you select an area you don't want to whiten, you'll also find a Quick Selection tool with a – next to it. You can use this to click on, and unselect the areas you didn't mean to select.
Once you're happy with your selection, go to the bottom of the Layer Panel and click the black and white circle. In the menu select Hue/Saturation. Adjust the Hue and Saturation and lighten the image until you're happy with the result.
Once complete, zoom out and you'll see your completed, smile-perfect image.
See the before and after shots here: