ERPNext requires MariaDB.
The Easy Way: our install script for bench will install all dependencies (e.g. MariaDB). See https://github.com/frappe/bench for more details.
New passwords will be created for the ERPNext "Administrator" user, the MariaDB root user, and the frappe user (the script displays the passwords and saves them to ~/frappe_passwords.txt).
You can download a virtual image to run ERPNext in a virtual machine on your local system.
System and user credentials are listed on the download page.