High-key photography, like low-key, is distinguished by intense contrast, and both methods are extensively used by photographers to enhance the drama of a scene.
In low-key imagery, black is the key accent colour, giving darker images with a more sombre feel. You’re right that this drama is usually achieved at the shooting stage, by setting up key lighting to the side of the subjects and bouncing the light around the scene in the right way.
Open your photo in Photoshop, then duplicate the Background layer. Desaturate your new layer, then go to Image>Adjustments>Curves.
You need to play with the curves in order to get the best contrast between the shadows and the highlights. As we’re going high key, you want to overexpose the image and go bright white in the highlights.
Duplicate this layer and then run a high-value Gaussian Blur (around 25-30px). Set this layer to Overlay to finalise the effect.
You will need to change the settings that you use for each image, as the Curves values will depend on how much contrast there is in the original photograph.