Email firstname.lastname@example.org or ask someone at your institution to add you as an instructor or TA to an existing course. Once you have instructor or TA access to an existing course, you can create your own course(s), add students, create assignments, etc.
If your instructor account gets created using a different email address than the one associated with your student account, you can always merge the two. To learn how, see the How can I merge two accounts? FAQ.