CREATE DATABASE

(1) you must be logged in as SYS with SYSDBA privilege invoked.
(2) you must have edited initmynewdb.ora file to spawn an oracle instance
(3) and you started the oracle instance in nomount mode.
(4) after the database is created you are also to create the medata and plsql packages
Example

connect / as sysdba
startup nomount pfile=$ORACLE_HOME/dbs/initmynewdb.ora
CREATE DATABASE mynewdb
USER SYS IDENTIFIED BY pz6r58
USER SYSTEM IDENTIFIED BY y1tz5p
LOGFILE GROUP 1 (‘/u01/oracle/oradata/mynewdb/redo01.log’) SIZE 100M,
GROUP 2 (‘/u01/oracle/oradata/mynewdb/redo02.log’) SIZE 100M,
GROUP 3 (‘/u01/oracle/oradata/mynewdb/redo03.log’) SIZE 100M
MAXLOGFILES 5
MAXLOGMEMBERS 5
MAXLOGHISTORY 1
MAXDATAFILES 100
MAXINSTANCES 1
CHARACTER SET US7ASCII
NATIONAL CHARACTER SET AL16UTF16
DATAFILE ‘/u01/oracle/oradata/mynewdb/system01.dbf’ SIZE 325M REUSE
EXTENT MANAGEMENT LOCAL
SYSAUX DATAFILE ‘/u01/oracle/oradata/mynewdb/sysaux01.dbf’ SIZE 325M REUSE
DEFAULT TABLESPACE tbs_1 datafile ‘/u01/oracle/oradata/mynewdb/tbs_1_01.dbf’ SIZE 325M REUSE
DEFAULT TEMPORARY TABLESPACE tempts1 TEMPFILE ‘/u01/oracle/oradata/mynewdb/temp01.dbf’ SIZE 20M REUSE
UNDO TABLESPACE undotbs DATAFILE ‘/u01/oracle/oradata/mynewdb/undotbs01.dbf’SIZE 200M REUSE AUTOEXTEND ON MAXSIZE UNLIMITED;

@?/rdbms/admin/catalog.sql
@?/rdbms/admin/catproc.sql
— then create the required tablespaces
— then create users and assign quotas on the tablespaces to all those users.