first you login sysdba user from sqlplus open cmd(windows os) or terminal(linux os) and type “sqlplus / sys as sysdba”
NOTE: SQL query is not case sensitive
STARTUP: when your database is compleate shutdown, using startup command you can start database is directly in open mode. Login database as sysdba you see that message “Connected to an idle instance” thats means database instance is compleate shutdown. in that time you cant not run any sql command.