Oracle Database Discussions

i want to create a databse manually in oracle 9i.The operating system i am working on is windows-xp.whenever i execute create database command --->>>
CREATE DATABASE mynewdb
USER SYS IDENTIFIED BY sonal
USER SYSTEM IDENTIFIED BY sonal
LOGFILE GROUP 1 ('c:/oracle/oradata/mynewdb/redo01.log') SIZE 100M,
GROUP 2 ('c:/oracle/oradata/mynewdb/redo02.log') SIZE 100M,
GROUP 3 ('c:/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 'c:/oracle/oradata/mynewdb/system01.dbf' SIZE 325M REUSE
EXTENT MANAGEMENT LOCAL
DEFAULT TEMPORARY TABLESPACE tempts1
TEMPFILE 'c:/oracle/oradata/mynewdb/temp01.dbf'
SIZE 20M REUSE
UNDO TABLESPACE undotbs
DATAFILE 'c:/oracle/oradata/mynewdb/undotbs01.dbf'
SIZE 200M REUSE AUTOEXTEND ON NEXT 5120K MAXSIZE UNLIMITED

i get error--->>>
CREATE DATABASE mynewdb
*
ERROR at line 1:
ORA-01501: CREATE DATABASE failed
ORA-01100: database already mounted

I got something on net like first create a new oracle sid by ::set oracle_sid={sid}.......then create a password file etc...............
but i am not getting where to perform these command.please tell me in detail what to do.