Are you looking to install a forum someone else has already written, or build your own from scratch? If you're looking for an out of the box forum, I would suggest installing phpBB from the app gallery. You can get there in WebMatrix by going New -> App Gallery -> Search for phpBB.
If you're looking to build one with ASP.NET Web Pages from scratch, I would suggest starting with the docs here: http://www.asp.net/web-pages
A great example to follow for this path is here: http://www.webpagescms.com/
Hope this helps. Happy coding!