Content is king, believe it!
Why did you come here? because of an ad ? nope, because of the great content.
Why would someone go to your forum ? because of your great ads leading them there ? nope, because you offer something special.
Personality, being first in a subject area, free stuff, being the official site for something, being an authority figure in your given topic... theres more but those are good reasons people will visit. What are you offering and do people want it? if not, no ads will help. Word of mouth and search engines work better than advertising but it takes work to earn those.
Ads make the world go round AFTER you make the world (cyberworld in this case). The thing is those ads will lead OFF your site and bring you cash to pay for your costs.
You don't have to be the best, just better than the competition and it doesn't hurt to have something original. Copying the next guy rarely works for long. Caring about the subject your site is about, being knowledgable, helpful and friendly, providing a needed service etc... those are worth more than any advertising in the long run.
I recommend google adsense for starters, its fast, easy and highly targeted which gives you more time for content. When you're established people will contact you and ask to pay you to advertise with you.