Press CTRL + ALT + F1
then at the login: prompt enter root and its password.
You can also use GDM/KDM to login as root, provided that GDM is configured to allow root login.
You may also use su to simulate a root session, Or you may also ssh to localhost as the root user.
You may also want to use sudo utility to be able to perform root's duties as a normal user provided that you have added yourself to sudoers file.