DerangedHost By default (at least on debian based), a apache2 install will utilize the /var/www/index.html file which you see to be the "start page". By default, /var/www serves as the "root" folder, equivalent to the top of a domain. For example in this scenario, /var/www/candy would be equal to domain.tld/candy
If you want flarum to appear at the same place as the "start page". You would consider removing that index.html files and installing flarum in the same place.
There are more steps involved in setting up flarum, including setting up a database, creating a apache config file for your flarum install (as opposed to the default one), changing permissions for directories (which the install should give instructions for)
If you're new to setting up a webserver, I would suggest looking up documentation on exactly that. Others have written far more extensive and illustrative guides then anyone could quickly create here.
Setup Apache2