Submit site to directories. Add url to forums and sites with same topic. Let people add url in signature.
Try to get as many backlinks as possible. Join forums with similar topic and add your link in your sig. Try to post frequently on your forum to show activity.
Hello , First of all you need a good design to attract people who are randomly surfing. Second you need to start advertising in any oppurtunity you get. SEO, Directories, Forums, Banners, Links. ( and so on ) The most important is to get active members , most simple way in my eyes is to hire some posters ( 0.50 cents per/post ). Offer freebies something like that , so users can get something they need from your forum for free. Be friendly - make an indrotuction forum and greet all new members. Few basic tips. I hope it helps you.