Default passwords of Oracle 11g?

On Windows

sqlplus "/as sysdba" 
or 

On linux/unix
sqlplus '/as sysdba'

type the command "sqlplus /nolog "
SQL> ALTER USER username IDENTIFIED BY "mypassword";

Once logged in successfully you can change password for the users created and use orapwd utility to generate a password file for sys/system user

orapwd example
Create a new password file: 

orapwd file=orapwSID password=oracle entries=5
If the password file already exists: 

orapwd file=orapwSID password=oracle entries=5 FORCE=Y