A forum is a bit like a party where no-one knows anyone. At first everyone is a little shy of posting comments etc. Once you get a few people posting interesting stuff the thing will tend to snowball.
One way of getting the ball rolling is to start a few topics yourself (preferably under pseudonyms). Try to imagine what topics are likely to run for a while, and make sure there are a few provocative ones.
Think about the first edition of a new magazine. They always have letters on the readers' letters page, even though they've never had any readers!
Ease of use is also important. We used to have a fairly active forum on a charity site that I run. When the (free) hosting company that we used went out of business, taking all our postings with it, we switched to another system, and it's never recovered - partly, I'm sure, because it's less intuitive to use.