You can subscribe to a topic to get updates on it. Otherwise I'd just keep a notepad file or set a bookmark with the URL to the specific thread you want to reference/follow.
At the bottom of the page on any post is a button which says "bookmark"
If you click that button, you bookmark the post to your account to save it for later.
To view the posts you have bookmarked, you need to go to your user control panel.
User control panel -> Overview -> Manage bookmarks.
Additionally, you can just bookmark the page with your browser. (sometimes called favorites) This will store the saved link to your browser as opposed to your account here.