I was going to suggest looking it up
here, but it doesn't appear to have non-base man pages...so I started digging around on one of my systems looking for the info, but "kdm" doesn't appear to be a separate package...so it may or may not have a man page anyways.
I haven't used kde since ~2005...so I'll have to defer to other forum members on this =) but the answer is most likely going to be "install and configure a graphical login manager".