Luna said:
What do you need to start a forum website? I'm curious.
And money wise, how much is it?
Assuming you don't want a free forum, a domain name and a host that provides script access (never buy a domain name with your hosting provider). LAMP (Linux-Apache-MySQL-PHP) is common, but not the only game in town. Dreamhost works okay for small to midranged communities, for $9 a month or so. That sort of price is typical for such places. They often have something insane like averaging less than 1% of the CPU for one minute rule.
As Death Angel Vox said, "A huge fucking load of people." Unless you have something to offer personally (see, for example, Dwarf Fortress, etc) momentum matters . Frequently a large enough community such as Elliquiy or Blue Moon becomes its own draw, but until that point, you need something else. Elliquiy came to be because RPoL became more puritan (and still is), sending wave after wave of people E's way as games got shut down. I'm not sure how or why Blue Moon began, but I made sure everyone who signed up to the old one knew about the new BM's existence to make sure it had momentum.
Starting a forum is pretty simple, at least until it gets big. Then you run into efficiency and scaling issues, eventually. You either need to know how to handle that, find someone who does, or get a 'real' solution - a VPS, a dedicated server, etc... and you end up paying say, $150/month for your second home instead of $10 >_>
What else you might want factors in, of course.