You'll find that it's already on the Internet
My forum has been running for six months now, and I've found the hardest part is getting new members - I advertise and advertise on other forums relating to the same hobby, you really have to work to get them to join. Plus when they do, you also need to want to make them stay - otherwise after joining they'll never log back on! You need to make sure theres something to make everyone stay and join in!
Make sure your forum has a real purpose too - for this one, it's Support, and look how successful it has been with so many members and posters. Make sure your forum has a purpose, and then advertise it, and you're good to go!
Good luck with your forum!