There are three main ways you can follow a comic:

If you don't have an account on The Duck, you can use the RSS feed for each comic to add them to your RSS reader. An RSS link is found in the topbar of the comic page:

If you do have an account on The Duck, you can use the Favorites system which lets you track comics on The Duck and when they last updated. When you are logged in, click on "favorites" in the topbar. A menu will drop down where you can add the current comic to your Favorites:

(Notice that if you have an account, you can also receive email alerts when a comic updates!)

Your Favorites list can be access either by clicking "favorites" in the topbar (as shown above) or in your account panel which appears on the Duck's main pages: