Hi Pauline, have you done a Google search like
http://www.google.com/search?q=greeting+cards&hl=en&start=10&sa=N to get some ideas?
I don't know of a site that shows you how but I'd start with a CD of clip-art, a graphics editor like Paintshop Pro and I wouldn't attempt it without a DTP app like MS Publisher or PageMaker...do you have any of these?
Or did you mean how to set up an on-line greeting card site? That would be fun too...