First, open your image (File>Open). Once your image is open go to the Select menu and click on Colour Range which will open a new window.
In this window you'll see your image, a series of sliders and some pipette tools. You need to select the pipette tool and begin selecting the colour you want to keep which in this example is the blue of the car. You need to click, holding shift on various areas of your image (the one you opened, not the one in the colour range box) until you're happy that the selection made shows all of what you want to stay in colour at the end. The selections you make are visible in the preview screen of the Colour Range box. Anything in the preview that is white will be selected everything else will not. To refine your selection you can turn the range and fuzziness sliders down. If you find you've selected too much you can also change the pipette tool to the one with a – symbol and remove some of the selections made.
When you're happy with your selection click OK and you'll see the areas you sampled from are selected with a line of running ants but this isn't the part of the image you want to be selected. For the next stage to work you want to go to Select>Inverse so everything else is now selected.
To turn everything but the chosen subject black and white go to the bottom of your layers panel and click on the circle which is half black, half white. From here open Hue/Saturation and very simply lower the saturation slider down to zero. Now everything apart from the subject (in this case the car) should be black and white. If you're not happy with the adjustment click on the black & white circle in the Hue/Saturation Layer and alter until you're happy. You'll also be pleased to know that your original image will also still be available in the layers panel.