The simple answer is that you've got to make it better than any of the several thousand existing forums that are aimed at the same audience. You can do all the SEO and social networking you like, but you won't get visitors to come unless you can offer them high-quality solid content that they can't get elsewhere.
I must warn you that you will be up against a lot of stiff competition. I'm not clear from your post whether you forum is concerned with marketing or with running a web site (you mention both), but either way there are a vast number of existing forums that you will be competing with (not least, of course, the one you in now).
You should do relevant forums,blogs posts.Also use social media sites for getting more traffic to your forum.Nowadays YouTube is also a big channel from where people are getting lots of traffic to their websites.