It is possible to connect to the database without specifying a password. Once you've done that you can then reset the passwords. I'm assuming that you've installed the database on your machine; if not you'll first need to connect to the machine the database is running on.
Ensure your user account is a member of the dba group. How you do this depends on what OS you are running.
Enter sqlplus / as sysdba in a Command Prompt/shell/Terminal window as appropriate. This should log you in to the database as SYS.
Once you're logged in, you can then enter
alter user SYS identified by "newpassword";
to reset the SYS password, and similarly for SYSTEM.
(Note: I haven't tried any of this on Oracle 12c; I'm assuming they haven't changed things since Oracle 11g.)
Login into the machine as oracle login user id( where oracle is installed)..
Add ORACLE_HOME = <Oracle installation Directory> in Environment variable
Open a command prompt
Change the directory to %ORACLE_HOME%\bin
type the command sqlplus /nolog
SQL>connect /as sysdba
SQL>alter user SYS identified by "newpassword";
One more check, while oracle installation and database confiuration assistant setup, if you configure any database then you might have given password and checked the same password for all other accounts.. If so, then you try with the password which you have given in your database configuration assistant setup.
actually during the installation process.it will prompt u to enter the password..At the last step of installation, a window will appear showing cloning database files..After copying,there will be a option..like password managament..there we hav to set our password..and user name will be default..