You've got a couple options, but no official forum package yet. The forum4umbraco project will probably be the definitive solution when it is ready, but no releases yet (
http://www.codeplex.com/forum4umbraco)
Beyond that forth-coming package you'll have to go down one of 3 avenues...
1. Pick your favorite forum... don't integrate with umbraco
Simply run the forum separate from umbraco. Lots of options. Just keep the url's from running into each other and you're up and running.
2. Make your own with AutoForm / Doc2Form.
This is how the original umbraco forum was built. You'll probably want to include some Event Handlers and maybe some custom .net controls to give you full forum functionality but you could get a long way with just the AutoForm or Doc2Form package and some xslt.
3. Integrate an existing .net forum package.
Depending on the forum package this can be quite easy to do. I've recently followed the advice of two trail-blazers and put a Yet Another Forum (
http://www.yetanotherforum.net) into an umbraco 3 site. Took about 1.5hrs from download to integration. Guides and advice at
http://skiltzonnet.blogspot.com/2008/02/integrating-yetanotherforumnet-into.html and
http://www.createsoft.co.uk/blog/?p=18cheers,
doug.
MVP 2007-2009 -
Percipient Studios -
Percipient Blog