well the answer to your questions depends in a couple things.
What 'logging manager' are you running by default ?
if you are running kdm, then you have there an option to chose the desktop of your choice (there are like 7(?) posible desktop enviroments aviable to linux)
Regardless (since I asume you have no idea what logging manager you are using), you can follow this instructions to jump to kde or gnome:
when you boot your PC, and reach teh logging screen (I assume a graphical interface is installed by default).
logging as 'root'
you should be in the comman line then... type:
~# killall kde
~# killall gde
~# killall xde
(you dont need to type all of the above, only the one acording to the display manager currently running, but since you might not know wich one... just try them all three :P )
WARNING: now is VERY important that you EXIT as root. Very dangerous start the Xserver as root.
once you exited, logging back as your regular user. Then type one of the following:
gdm - for Gnome
kdm - for KDE
Disclosure: I am a NEWBIE too !! so if you get a better advice... be my guest !
If you run the kdm by default, you will by given the option to launch ANY desktop envirometn installed in your system at the logging screen.