Requirements for template operation:
1.Install Zabbix agent and MySQL client.
2.Copy Template_DB_MySQL.conf into folder with Zabbix agent configuration (/etc/zabbix/zabbix_agentd.d/ by default). Don't forget restart zabbix-agent. 
3.Create MySQL user for monitoring. For example:
CREATE USER 'zbx_monitor'@'%' IDENTIFIED BY '<password>';
GRANT USAGE,REPLICATION CLIENT,PROCESS,SHOW DATABASES,SHOW VIEW ON *.* TO 'zbx_monitor'@'%';
For more information read the MYSQL documentation https://dev.mysql.com/doc/refman/8.0/en/grant.html , please. 
4.Create .my.cnf in home directory of Zabbix agent for Linux (/var/lib/zabbix by default ) or my.cnf in c:\ for Windows. For example:
[client]
user=zbx_monitor
password=<password>