I have a succesful forum. I started with just 2 forum categories, the main general discussion, and one targeted at the site's theme.
I waited for the community to grow some before adding other categories.
Failed forums I have seen tend to start with a dozen or so forum categories, that's too many for a new community.
Also start right off the bat installing anti-spam mods because (drumroll) if you build it, they will come... Meaning the spambots... they are like alien rodents, or cyber fireants, whichever seems better... They find you and try to fill your new community with all sorts of garbage.
Choose a popular forum software like Vbullitin or phpbb they have a LOT of anti-spam and security mods available. Choosing an obscure forum software may keep a lot of the bots away becaue they arent writing bots for them, BUT if they do, you may find a total lack of mods for it to defend yourself.
Wait for the community to get well established and pick the most helpful, most productive active poster and approach them with the idea of being a moderator, on a trial basis.
Use some kind of reward system, kinda like here at WW where you get a different "rank" based on the amount of posts you make.
Don't make the newbie rank too insulting, like "noob" or "greenhorn" nobody wants to see their well thought out post as being authored by a "noob" lol... And on the flip side, don't make the reward for 5000 posts too grand or exalted like "Peerless Expert" or the like...