All below actions should be executed with "oracle" user account

1. Check the status of listener
$ lsnrctl status


2. If the listener isn‘t be started, start it
$ lsnrctl start


3. Set the "ORACLE_SID" environment to the DB instance‘s SID which you want to start, for example
$ export ORACLE_SID=PC855


4. Start sqlplus and start the DB instance
$ sqlplus "/as sysdba"
SQL> startup


5. If you want to stop the DB instance
$ sqlplus "/as sysdba"
SQL> shutdown immediate


6. Check the DB instance status again
$ lsnrctl status

Notes:

1. Make sure the listener be started before starting the DB instance
2. Make sure the ORACLE_SID environment has been set correctly
3. To configure the listener
$ netca
or
$ netmgr
4. To configure the DB instance
$ dbca