If you need to install MySQL for what ever, you will have to launch this script :
mysql_secure_installation
The first question is to choose a password but since it’s the first installation, you didn’t choose any ! If you installed mariadb you can « bypass » this step by pressing enter but if you installed mysql-server you have to check in the log file created by mysql-server where you can find a temporary password.
Something like that : cat /var/log/mysql.log