In a nutshell - Open the image you wish to make duotone, tri-tone, quad-tone, whatever. You will need to convert the image into greyscale. While doing this IMAGE > MODE >GREYSCALE, you will no doubt see an option for duotone. Once you have the image in greyscale, you will be able to go IMAGE > MODE >DUOTONE.
A dialog should open. See if you can get that far. If so, tinker. If still confused, say so, and I'll gladly post further info.
http://www.google.com/search?hl=en&q=creating+duotones+in+photoshop&btnG=Google+Search