What if I decide to take one more course?

Since you have already registered, you can login in and click on Course catalog from student tools on right side. It is the last one down and select the course from the list like before.
Or, from login screen you can click on course catalog and select the course and you want to register for and follow the same steps. If course needs approval from administrator then you will be notified about it. In that case you will have to wait till administrator approves you. You will be notified of approval. Once approved you will be able to see the course in "My course" list and then you can use it.