If I may be so bold as to add a few details
on a UNIX server you can just fire up sqlplus (or svrmgrl for older versions of Oracle) and issue the create database statement.
On a Windows server you must first create the service with oradim. For example:
%ORACLE_HOME%\bin\oradim -NEW -SID %ORACLE_SID% -startmode m -pfile D:\Oradmin\%ORACLE_SID%\pfile\init%ORACLE_SID%.ora -intpwd my_secret_password -maxusers 4
-startmode m means startmode manual - the instance must be manually started after system reboot.
-intpwd is the password for the sys user
-maxusers 4 is the maximum number of users that can be given SYSDBA capability
Once you've created the database and all the tablespaces, you may want to do the following to avoid annoying warnings everytime you startup SQL*Plus:
create PRODUCT_PROFILE with this script
$ORACLE_HOME/sqplus/admin/pupbld.sql
(I usually create it in the SYSTEM schema)
Create Plus_Trace role
$ORACLE_HOME/sqplus/admin/plustrce.sql
To install SQL*Plus help, look for a file called
%ORACLE_HOME%\bin\helpins.bat on windows
$ORACLE_HOME/bin/helpins on UNIX
Finally, don't forget to change the passwords for SYS, SYSTEM, and other users automatically created by the database creation scripts. And change the TEMPORARY tablespace for users to something other than SYSTEM.