Well, tricky question. Many years ago, starting a forum was a breeze. Now, after years of social media, it's getting rough. The upside is that, after a decade of destroying forums in the SERPs, Google is not favoring them again. So it's a good time actually to grow communities.
As for starting it up: a few tens of decent threads, paid posting, exchanges etc. Of course, if you have the money, PPC can also speed up the process.
Always offer something: a tiered membership system (people love it), prizes for the most active ones, access to free stuff (that's exclusive to members) etc.