Well, I"m sure there are a thousand and one tutorials out there, I'm sure you can find the information you need on this website itself. But as we speak, i'm making a forum for my website. Very simple really. all you need to do is make a mysql (if that's what your using ) table that will hold the forum information, like who created it, title, date, message, whatever
another table that will hold the replies from the forum. Link them through the same forum id number.
Then it's simple write and recall functions you make. Make a script to let users creat a forum, reply to a forum, and simply look at a forum. I know i generalized a lot, but it's not as hard as it seems nor as easy as I made it to be. Also, there are alot website that create forums for you if you dont feel comfortable in php/mysql. hope this helps, let me know!