Layer Masks

Hi, I need help with layer masks. I've looked through the memories and found quite a few for PS. I'm using PSP9 and can't really figure out how to do it since I'm new to the program. I want to hide some part of the image. So do I add a new layer mask? What next? Or do I have to load a new mask? Can anyone please explain or point me to a couple of tutorials? All help is greatly appreciated.