Gretchen, its quite simple.
Your image resides on one layer, you're going to create a BW layer over that one using the Channel Mixer. On the layers pallette, you will see a row of icon sat the bottom of the pallette, the fourth from the right is a circle half filled with white and half with black, click that and the select the Channel Mixer adjustment layer.
You can tweak your adjustment layer by sliding the red, blue and green slider to get your desired B/W image.
Once you have that, you will have a color image covered up by a BW adjustment layer.
Now for the fun. On the pallette menu, select the third icon...its a mask. Make sure you have your Channel Mix layer selected first though. This will add a white box next to the icon in the channel mix window.
In order to reveal the color beneath you have to click in the white area of that window, then begin painting with a black brush over the parts you want to be revealed. You can use a wide brush set to a low opacity...if you use to much, switch to a white colored brush. White conceals, Black Reveals.
Zoom in on your subject that you want to paint and slowly reveal it.
Does that make sense? I'm reciting it to you from memory....
