It's pretty difficult to get a forum going about something that already has a lot of discussion groups about it (ie computers or gaming). Your best bet is to find something unique or that isn't real popular and then find members of love a similar crazy hobby or something. If you're set on the topic of your forum, getting friends and people you know in real life who are interested is the best way to get it off the ground. Ask them to be active and spread the word to all of their friends.