Most important is to select what your forum is about, try to keep it focused, choose something that interests you personally and make it appealing, pertinent and interesting for participants. Also you need to be visible to make it work and you can use the forum or blog directories on-line that will list your site (often for free with a reciprocal link). It will take time so don't expect it to happen overnight- some forums have been going for years but a few months at least to get it off the ground.
If you have a passionate interest in the subject matter it will really help drive your project along and keep motivation and morale high.
Send me the details and I'll join to see what it's like - that's always a good way to get people involved - just ask.
I wish you the best of luck and continued patience to see it through - please do let me know and I'll visit