Is another webserver already running on your server? Keep in mind that only one process at a time can listen on the HTTP port. If you’re already running e.g. Apache with name-based virtual hosts, you’ll have to configure your existing Apache to act as a reverse proxy for Discourse and change Discourse’s config to expose itself on some unused port, ideally bound to localhost.
There are a few topics on meta covering this, e.g. Run Discourse with or alongside existing Apache sites? and the howto Running other websites on the same machine as Discourse (which focuses on using nginx as the global frontend server).
Also, mind that Apache in its typical default configuration isn’t well suited to host Discourse. You absolutely should run with mpm_event and a large AsyncRequestWorkerFactor setting or Apache won’t handle Discourse’s many long-running polling connections very well.