Log on to mydb database

From command prompt (Not SQL*Plus) type the following;

set oracle_sid=mydb

sqlplus “sys as sysdba”  

(No password is required if you logged on to  the server as user with admin rights; In that case just press the enter key when asked for password)

 

One time setup to change user passwords 

SQL> alter user hr identified by hr account unlock;

SQL> alter user scott identified by  scott account unlock;

SQL> alter user system identified by system  account unlock;

SQL> alter user dbsnmp  account unlock;