Tutorial: http://www.phpeasystep.com/phptu/12.html Although i'd recommend before you use that tutorial to learn atleast the basics of php (if you haven't already), so you can get a better understanding.
Forums are a HUGE PHP project. It is not something you just decide to make one day. It will take a vast PHP and MySQL knowledge and a bit of JavaScript wouldn't hurt to know either to make a couple little functions on the site. I would recommend you learn PHP well and starting working on smaller projects and build up to finally making a forum. In any case, good luck!
The task is really cumbersome and after the initial release many bugs will appear on a daily basis! So I'd recommend to go for popular and well known forums scripts that have matured over the years like phpbb, smf or mybb! good luck
forums are really huge projects, and so many forums are available few good are paid and free also available like phpbb. but there are so many things on those forums, if your requirements are simple, then really the best way is to develop ur forum yourself.. would be best..