That's the default way to view it, so you may have changed an option somewhere along the line.
There's two places that may help you:
1)
Edit your profile options and scroll down to "Thread Display Mode" and set it to "Linear - Oldest First".
2) and for the thread listing in a sub-forum, scroll down to the bottom of the page and you'll see "Display Options" - you can change the sort order there.
Hope that helps.