It's difficult with warez sites -- as it is most definitely illegal. Your traffic will most likely strike success by backlinking other warez sites, and SEO. Ads...unlikely.
Its same technique with promoting normal forum. Such as creating campaign by offering incentive for member who able to recruit new member to join your forum.
a) starting a forum is one of the hardest things online, you will need a lot of dedication and you will have to post every day into an empty forum b) you need to do a lot of events, giveaways, contest to attract people c) considering your question I suppose you are new to web developing, therefore I'd recommend to start with blogging instead of starting a forum, then you can go from there and build up a community that is driven by the main blog d) why warez? change your niche, you will get into a lot of trouble .. if you want to get in debt then feel free to do it, but instead you could start a blog about filesharing services (just an example) then you could hire a developer and make him create a unique uploading service .. then you will start your own uploading service monetizing the traffic you receive to your blog