[Skunkworks] Photoshop Help

Hello Skunkworld, I'm trying to create a cutout shape in photoshop....basically I have a shape on top of another shape. What I want is to make the bottom shape have a hole in it shaped like the top shape. It's like a stencil...like what we used to do in Art & Craft in primary school. I hope I'm making myself clear... Picture a CD, you know, the hole in the middle of a CD? Something like that, only now in an image, and it's not a circle so I can't use the elliptical marquee tool. And there is no "Custom Shape Marquee Tool", at least not in the version I'm using. Google results don't have what I'm looking for. I'll really appreciate your help.