After you complete your payment, you'll be taken to the login page and asked to set up your username and password. You can then login right away and see your course.
You'll also receive an email with a reminder to set up your login if you don't do it right away. So, either way, your login will be available immediately after you enroll.
Once you're inside, all you have to do is click on your course (it will be shown as an image and the course title) and you'll be off and running. The course is laid out very clearly and visually so you'll know exactly how to proceed.