There’s the standard link at the top of each thread that will take you there, but there are a couple of other links:
On the thread view page, there’s the icon just to the right of the avatar that will take you to the first unread post, as it will tell you if you hover over it:
And somewhat confusingly, if you click the bold red title it will take you to the first unread post as well. This part:
It makes sense, and is probably the outcome you want 98% of the time, but in those cases where it isn’t and you find yourself at the bottom of a thread where you really want to be at the top, there’s this link at the bottom-right of the page:
Click that and you’ll be at the top of the page.
And as an aside, see the arrow pointing up and to the right just above the Top link in the above photo? Clicking there takes you to the quick navigation menu, so this will pop up: