Well, I'd choose your ideal topic and then research your audience.
See if they're on forums. If not many are using forums, I find it's a lot harder to convert them. That doesn't mean you can't, but it will just require more pushing on your effort.
Make sure you have plenty of content each day if you can do it. Post a few topics a day and answer any posts once you start getting more people.
If and when you start a forum, let me know, I'll definitely join and try to help give you some content :)