Ir, if you're talking about seeing threads with new posts listed at the top of each forum, then as Roger said that's already the case by default.
If you're talking about seeing threads start with the newest posts first, well, at the bottom of every thread you'll see the following controls:
Display posts from previous: [b][All Posts] [Oldest First] GO[/b]
Change "Oldest First" to "Newest First" then click the go button.
I
think there isn't currently a way to make that setting permanent, though.