Tutorial: Curves

Curves are yet another way of changing the contrasts and brightness of an image, similar to Levels, but with much more control over what you are doing. Most people start off using Levels and gradually progress to using Curves to manipulate their images. Once you get the hang of it you might not want to use any other method.

Step One:

Open your image in Photoshop, then go to "Images", then "Adjustments" and select Curves.