personally I would use a predeveloped forum and modify from there. There are many free forums out there such as phpBB3
There is actually a good tutorial on youtube... it's by a fella named lifegoeson666... aka marcus. Here is a link to the entire playlist of videos in the forum creation series http://www.youtube.com/view_play_list?p=7DC9C3BA270799EF good luck ^^
I think there are many open source forums you can use them like phpBB and many more, but if you really want to make you own then i think you have to work very very hard on that , because people spam them ! Thanks,