Welcome Nick!
I agree with Sherif. I tried to launch a forum numerous times when I first started. I failed each time because I had no traffic. You first need to build a site that can generate traffic and then launch a forum.
Let's say you wanted to build a community for teens to chat. It would make sense to build a content/information site with articles dedicated to teens. Perhaps you'd have articles on self esteem, peer pressure, etc. The reason being is that it's much easier to get free search engine traffic to your site through a content site than a forum. So you would use that traffic to promote your forum and encourage activity.
I use my site 2createawebsite.com to promote this forum and that's how 90% of the people find it. But 2create had thousands of hits per day before I even launched websitebabble.com
Many people put the cart before the horse. And starting a forum when you're brand new is definitely the cart in this scenario. First, build a content-rich site on the topic you're interested in then read up on search engine optimization (How to make your pages rank high in the engines).
www.selfpromotion.com is a great place to start.
Hope that helps!