See the attached file and play with to see how it's done, in a way which is reversible/adjustable. You can directly saturate/desaturate the image layer, but it's a pain to undo/adjust later on, and you can't turn it on/off like you can when you use a layer mask.
A method using layer masks as mentioned is the best way. You create what's called an adjustment layer, in this case one which controls hue/saturation/lightness. Playing with the sliders will affect the whole image.
What you do here is, while you have the adjustment layer selected, you click on a little button called "add layer mask" on the bottom of the layers pallette. This creates something similar to a stencil mask for a spraypainter. Drawing in black or white using your brush tool, adds and subtracts to your "stencil mask".
The black subtracts, and the white adds... kind of like, black is deleting the saturation adjustment layer (letting you see the layer below - in colour), and when you make a mistake, brushing in white on the mask will 'undo' what you've done by adding back to the mask.
I couldn't stand learning it in the beginning because there were all these little things to get used to, but like everything, you get used to it.
When you open my file, select the layer called "mask", by left clicking it, and then select a white or black brush and see what happens when you paint over the b/w and colour areas.
A handy shortcut for layer masking to speed things up;
* "X" switches between black and white, so you can quickly switch between adding and subtracting to the mask.
* "B" is for brush mode.
Note that the brush tool is only one way of creating and adjusting your mask. You can also use the vector tools if you like, and other quick selection methods which you can play with as well.
Have fun and let me know if any of the above isn't clear.
