Login to sqlplus session either on your command prompt or on sql*plus window using user "/as sysdba" or user SYSTEM.
If you are using user "/as sysdba" then it does not require any password. Check the screenshot below, just enter "/as sysdba" and click ok you will be allowed to login or user SYSTEM user and its password to login.
Once you have logged in issue an sql*plus command SHUTDOWN.
ORACLE instance shut down.
This shutsdown your database.
To start the database you need to login again as
"/as sysdba" and then issue a command STARTUP.
ORACLE instance started.
Total System Global Area 583008256 bytes
Fixed Size 1250284 bytes
Variable Size 213912596 bytes
Database Buffers 364904448 bytes
Redo Buffers 2940928 bytes
Screenshot attached for reference.
To know how to start/stop the database using batch files CLICK HERE
To know how to start/stop using services menu CLICK HERE