If teacher has created the course space to Moodle via Sisu, you don't need any course enrolment key. Course should automatically be in your Moodle My Courses page after registration to the course. If you cancel your enrolment in Sisu, it will remove you from Moodle course space.
Other than Sisu connected courses you usually enrol in Moodle course with a course key. Teacher or administrative staff will tell you the address of the course and the course key for the enrolment.
Find the course in Moodle with Find in the header or browse it from the list of courses on your Dashboard, if the teacher haven't send you the link to the course via email. No enrolment key is needed, if course is connected to Sisu registration.