Hi
I have luanched my own Forum website for that visitor and traffic is required for free only
To get free traffic, you could use social media (especially groups), forums, classified, guest blogging and any place lets you post for free. Sometimes, only need to use 1 or 2 place is enough
You have to promote social media, build brand awareness of your forum and participate SEO Webmaster forums. Both will increase your traffic.
You should create a huge community across your new forum. Use FB, G+, Twitter and other possible social networks. Post quality content and your forum will growth by itself...