You can enrol into a course one of the two ways: through Premium subscriptions where you get unlimited access to every course, or by purchasing a single course just for USD$5.00. You may also purchase a course program which is essentially a course bundle. A program purchase will enable you to enrol a bunch at just one price. However, in any case you will need to login and charge your account with credits first.
The minimum re-charge amount is USD $5.00.