See this link:
https://www.phpbb.com/[
^] They are specialized on making freeware forums.
They have help and user forums also.
There are other options like that around:
http://www.codefear.com/scripts/5-best-php-forum-bulletin-board-script/[
^]
http://en.wikipedia.org/wiki/Comparison_of_Internet_forum_software[
^]
If you want to do it by yourself you'll have lots of work...
As a new approach after knowing that you want to do the job by yourself...
I would look at TinyMCE to allow the users to write their messages, MySQL to simply store all their HTML code, and establish a good database design to accept multiple inputs at the same time.
what you would like to do it is like a news page from most companyes but allowing that anyone could populate it...
Good luck!