1. You should have a blog or site that has a good amount of traffic. No one wants to just join a forum unless it's totally unique nowadays.
2. You need money to start up a forum just by itself nowadays. You will need to advertise through a program.
3. It takes many years to grow a forum. Freeones forum has been around for 7 years at least I think.
4. If you build it they will come... That's not true at all unless you have a name already.
5. Submit to search engines and niche specific sites. You can also use blog comments as extra ways to promote a site, but that takes a long time to build a decent amount of links.