Another method, other than Alin's, is to use an adjustment layer.
- Click on the " Create new fill or adjustment layer" button at the bottom of the layers panel (1) and select Hue/Saturation. This will create the new adjustment layer on top of your existing image.
- Slide the saturation slider (2) all the way to the left, to desaturate the image.
- Now that your image is black and white, click on the layer mask (3) and use a black paint brush to draw in the areas that you wish to restore color to.
Giving you something like this:
Since you are new to Photoshop and might not know about them, Layer Masks are an incredibly valuable method of "erasing" parts of layers without actually removing the pixels. A layer mask works by showing or hiding parts of an image, based on a grayscale mask (black = hidden) and can be edited in many of the same ways a regular layer can be. It is always a good idea to avoid erasing or deleting anything in Photoshop, if you can. Do this by using things like Smart Objects and Layer Masks.