Category Tags

I'm trying to add the tag to display the category of the entry in the posted by line at the bottom of each entry. I've looked through the help files and tried a couple of things that obviously didn't work. Tried to do a search in the sidebar but got blank page. Can someone help me out with the code, I know where to place it in the master index and I have the categories archiving enabled.

Thanks for any assistance you can give me!