This is done automatically.
When you click on "new posts" it is automatically customised for you. It actually means new posts, just for you, arranged in descending order from when they are posted.
If you click on a thread if you go back to "new posts" you will see the thread has disappeared from the list. The software
assumes you have read the posts so it no longer new.
When a fresh post is made in the thread it will reappear.
The user CP threads work the same way, but will only show threads to which you are subscribed.
Subscribing to threads is a great way to keep track of those threads that you find interesting.
Like new posts only those threads that have new posts since you last viewed them will appear in the list.