YOu can design the front end any way you want really as the functionalty really wont change. (Everything is a rudimentary call to a database)
You would need to set your database up with, at minimum, 4 tables. A users table so that you could validate users, A categories table that would contain all of your form categories, a threads table that is related to your categories tables, and lastly a comments table that is related to your threads table.
I am a big fan of not reinventing the wheel so check out www.phpbb.com
they provide free forums that run on PHP and MySQL
I will only tell you how to do it, not do it for you.
Unless, of course, you want to hire me to do work for you.
^^Thats my signature