There is a setting in your settings panel here, called Thread Display Mode which governs whether posts in a thread are displayed Newes First or Oldest First. So if you set it to "Linear - Newest first" you will see the most recent replies first on all threads on the website. This is how I have my profile set and I reccomend it if you use the website often.
If you have that setting set to Linear - Oldest First, which is the default, you will see a link on the top of the first post of each thread called "View First Unread" which does what it says!