You simply follow the installation steps provided on this page on the main site. Alternatively, do a couple of searches for discussions on installation here on the forum. There will be numerous discussions outlining what you have to do to get your own instance of Flarum up and running.
Bear in mind that Flarum can only be installed via Composer at the moment. There isn't a way to simply download and upload the forum software yet as you would find with other forums.
