You can enroll in currently available courses in just a few steps from the Class Central information page (e.g. one like this).
Scroll down the page a little, then click on the green "Go to class" button on the right side of the page. This will take you to the course page on its learning platform. If this is your first course, you will need to sign up or register with your name, email and a password. Some course platforms allow you to sign up via social media, or may ask for more contact information as well. Once you have signed up, you can click on the Enroll or Join course button to start learning.