1. It's best to do it after you start and build up a site with a large base. 2. Have a topic, or niche to associate your forum with. 3. Start small, not too many forums. People who see alot of dead empty forums tend not to visit too often. 4. Be unique, offer a feature that you don't see on many other forums.
PM me and I can help you get set up with a hosting provider that offers a 1-click forum software install, and get you a little discount on it too. After that, it's just a matter of finding a few interesting people to start posting... communities will follow when you get just a few interesting people as regulars.