I run a poker forum and I know just how difficult it is to reach critical mass - I came close in 2010 but never got there; however I learned a lot on the way. The two 'easy' routes to establishing a thriving community:
1. Attract thousands of visitors each day: add signup CTAs across the site, start a few threads and it will soon pick up.
or
2. Create a user focused forum with medals, avatars, competitions, ranking/reputation systems and even points that can be spent on unique forum items. Turn posting into a game and you will be rewarded - Google 'gamification'.
The only other way is a long hard slog that involves asking each and every person you know to help out whilst also creating dummy accounts and posting fake conversations each and every day without fail. Unless you have lots of traffic, or the budget for a great forum system, then unfortunately this is the only way.
All the best,
Nick