Emma,
Creating a forum in PHP is not very complicated, but there are a lot of things to think about. How will you be displaying your forum? Do you want messages threaded (like the tree format)? Who will be using your forum? Will there be more than one category to post messages into? How will you identify those that post messages? Will you be using sessions?
Two months ago I decided to build a webpage that primarily would be a forum. You can see the final product at www.AaronZoller.com. I am actually right now completing an article that will describe in detail the methods of building a forum. It should be done in the next day or so, and will be published at AaronZoller.com for you to view if you are interested. In the meantime, feel free to drop by and give it a test run. Then let me know what kind of things you are looking for and I will be happy to help. God I'm such a techie-nerd, it makes me sick!! :-)
AaronZoller.com