Okay so you would need to login to your hosting provider and create that a-record for the hostname you chose for flarum. If you intend to use the main domain you need a record for "@" and "www" pointing to the IP of your dedi.
You'll also need to set permissions and ownership for the web-serving directories. With apache this is usually www-data. Did you create a database in mysql yet? I'm trying to understand how far along you are.
