HTML, CSS, PHP and jQuery. With those four languages you can build anything.

Now, there are plenty of forums already out there, so you may want to consider extending a forum that is close to what you like with additional features via plugins.

If that's not possible, then take a look at a framework (Yii, for example), which should significantly speed up the build process, as it comes with tools any website/application will most likely need.

~Sovereign