You need access to the database, e.g. via an interface like phpMyAdmin. Ask your host about it.
If you host the forum yourself, as mentioning Yunohost suggests, you should install phpMyAdmin yourself (if it's not already installed by Yunohost) and point it to your database, which usually is located at localhost
.
The database should already exist, as this is a prerequisite for the installation of flarum. In the beginning of the installation process you had been asked for the database credentials (db name, user, password), the same settings you will need now for phpMyAdmin.