Forums that are large are very very time consuming and hard to run, if your not willing to sit at it for 12 to 14 hrs a day working at it then you will not benefit. You have to be what the members are looking for and with the online communities they are looking for someone to listen and advise them about anything and everything, you have to maintain a strong presence on the forum and just have the right personality I think that will not turn people away.
You have to start from the ground up, it is not easy, you will need to go and post all over the net in other forums( be sure to do it in the right spot so your not spamming) be friendly with other forum owners most are willing to advise you and there that are not so friendly.