Within the Streaming Marketplace there are multiple products to choose from, all of which you're able to learn more about by clicking on the button "Learn More." You can also visit the partner's website to get a better feel for the service before signing up.
Once you feel ready to purchase, simply click enroll and enter your credit card to create a monthly subscription. Once that's done, you will receive an email with your unique enrollment code and a link to where you'll finish the enrollment process.
You'll then be free to start taking classes!