at login, you can use 'root' as your login and then the password you entered when you installed mac os x.
as BlueFlameOut above mentioned, you can 'su' to root in the terminal. This is the better solution. Logging in as root increases the risk of damage to the system. Generally, the best strategy is to log in as yourself, and su to root as necessary. Otherwise, you could risk permanently damaging your system, and no one wants that. Especially if you are new to unix.