How do I resolve this "ORA-01109: database not open" error?

I'm trying to create my own database using SQLPlus. So first I log into it as admin:

sqlplus sys/sys_password as sysdba

And then I try to create a new user, called sqlzoo :

CREATE USER sqlzoo IDENTIFIED BY sqlzoo 
DEFAULT TABLESPACE tbs_perm_01sqlzoo 
TEMPORARY TABLESPACE tbs_perm_01sqlzoo 
QUOTA 20M ON tbs_perm_01sqlzoo;

This gives me the following error :

ERROR at line 1:
ORA-01109: database not open

Why is it giving me such an error?

As the error states - the database is not open - it was previously shut down, and someone left it in the middle of the startup process. They may either be intentional, or unintentional (i.e., it was supposed to be open, but failed to do so).

Assuming that's nothing wrong with the database itself, you could open it with a simple statement:(Since the question is asked specifically in the context of SQLPlus, kindly remember to put a statement terminator(Semicolon) at the end mandatorily, otherwise, it will result in an error.)

ALTER DATABASE OPEN;