If youíve already installed Debian and you want to add the full KDE environment, then open a console, change to root and type;
apt-get install kde
apt-get is Debianís package manager. The install part tells apt to install a new package and the KDE part is the package you want to install. This will be the FULL KDE environment which comes with loads of extras. I often find itís better to have a basic KDE setup and add new bits as I need them. For that do.
apt-get install kdebase
If you really have no interest in Gnome and would rather get rid of it you could also type
apt-get remove gnome
Similar to above, but this time the action is to remove a package and the package is Gnome.
Lastly, if you feel more happy with A GUI then with a command line, you might want to install KDEís graphical package manager. Try
apt-get install kpackage
Now you can add an remove programs from within KDE.
If you're new to Debian then you might want to look at this thread;
There's some good information and links in there.