you really need to find the shall we say it 'newer' internet users - someone with a small amount of interet knowaldge would know how to get a free host and upload a forum, if you find people that know almost nothing about how this kind of stuff works they will obvioulsy want to use your service, now you knew this already but where do you find these people?. . . . first of all are you a member of any large forum where people might be looking to start their own forums, gaming forums maybe? 'new' gaming clans often use this type of service for their clans forums so if you can advertise on any gaming/clan forums that could be a place to start for you, another place could be a sport community people might want to start a forum for them and friends to talk about whatever sport/team they follow

try to think of as many places like this as you can and you should be able to bump up your user base quite a bit from that