You can sort comments on the Duolingo forums or in Sentence Discussions by selecting "Sorted by ..." on the top-right hand corner, right above the first comment of the thread.
On regular discussions, the default view is "Top Post" and in Sentence Discussions, the default view is "Top Thread." You can use the drop-down to select how you want to sort the current discussion. Here is how sorting works and what each option means:
Sorted by Top Post
This is the default on the forums. In short, it is sorted by upvotes on "parent" posts. The parent post (a top level comment, or direct reply to the original post) with the most upvotes will be on top, and it's "child" comments will follow, no matter how many up or down votes they have. The next thread to follow is the second most popular "parent" post, and so on.
Sorted by Top Thread
This is the default on Sentence Discussion posts. The whole family of comments (a "parent" comment and its "children") is called a thread. So if you sort by Top Thread, it will take the entire thread's up and down votes into consideration. This is typically good for Sentence Discussions, as many parent comments are questions, and people tend to upvote a helpful answer more than the question itself. This way it is easier to find popular answers quickly when you are in the middle of your lesson.
Sorted by Newest
The most recent top level comment will be first (along with its sub-comments), followed by the second most recent comment, and so on.
Sorted by Oldest
The oldest comment will be first, and the second oldest top-level comments next, and so on.