Several ways:
- You can grab a terminal (which you can find in the menu, apps, system), and type su, when it asks for a password it's your rootpassword.
- You can leave the x-server by Ctrl-Alt-F1, you'll get a console. You can go back to X by Alt-F7
- You can configure gdm with the program gdmsetup (so you can login as root with the graphical login), but you must be root to configure it.
- If you don't want X to start at boottime, delete the symlink S99gdm from /etc/rc2.d, but you must be root to do that
.