It's a community forum, so you need to have something to say and to know that the gap you're targeting exists.
In two parts this simply means:
1) You had better be interested in the topic
2) Check which other sites already exist and position yourself accordingly
My most successful forum is LFGSS http://www.lfgss.com/ , which is London Fixed-gear and Single-speed... a cycling forum which luckily caught a trend before it got big. The positioning on that was that whilst other cycling forums do exist, others failed to recognise that cycling is a local activity and that cycling itself splits into various tribes and that even local activities only have appeal to one tribe. I love cycling, and I was riding fixed, so once I recognised that I could just start the London forum I was pretty much done.
To launch it I just went on to sites like bikeforums.net and announced in the most applicable place that I was starting something I hoped would appeal to the niche I'd highlighted. About 15 people signed up on day 1, just enough to have conversations between each other... and with about an hour a day invested in chatting and going on the odd ride I get it to the point of getting 30 > 50 new users every day and am now somewhere over 25,000 members with over half of them actively using the site each month and a unique visitor count (Google Analytics) of 250,000 per month.
My other forums aren't doing as well, but all produce revenue and the platform is already paid for by LFGSS.
It took me a while to work out the revenue model (get users first!), but once worked out they're doing fine.
Seriously though... I would look at eBay and find high value items (above $1,000 USD average) and ask myself which of those is something I'm interested in and could invest in for the near future (as in... treat as a hobby). Then create the site, and instead of thinking about the revenue and site, become a user and just contribute as you would if you were a user. The money follows that.
