Before you can actually earn big with your forum, you should first be able to draw a lot of traffic and members to it and encourage them to participate regularly. Remember to provide useful and informative content so as to entice your traffic to sign-up and participate.
You can start by building quality links to your site and promoting it through the following:
Directory Submissions - targeted links at high PR directories
Blog and Forum Postings - to high PR and frequently crawled sites
Social Bookmarking
Classified Ads
Article Marketing - submit to high PR directories, could also serve as link baits
Link Exchange - with relevant and authority sites
Link Baits - for organic linking
Etc. etc.
Monetizing it will be a lot easier once you have established a good member base
