I would highly recommend following a few learning paths in the Academy to get used to domain models and how to set them up. In this case you could make the association a many-to-many association so a course can have many students and a student can have many courses, however, in most cases best practice is to set up an entity in between. You could call it something like “Registration” and have a many-to-one with Student and a many-to-one with Course.