Three important things:
1.A webserver, either IIS or Apache.
2. Forum software. vBulletin, invision power board, phpbb are some of them.
3. A server to host the same.
Other things:
1. More members
2. Good content, etc...
Most paid Linux hosting packages comes with phpBB installed. All that you need is a browser to install and configure it. Even this forum is a phpBB forum. But remember one thing. If you have good number of visitors to your forum, you will require a lot of space and a lot of bandwidth.
But take a look at *www.proboards.com. May be you don't need to spend even on hosting and domain
Hi Guest we just wanted to alert you to a major change in the forum. We will no longer be allowing the posting of outgoing links. Please use the attachment feature to attach media to your posts.