Course enrollment without SITS enrollment
Most of the time, you will not be able manually enroll yourself on a course in Moodle, you have to register in SITS.
In case this option is available, you can enroll to a course without a final commitment in Moodle, if your professor allowed this. So you can enroll, check out the course details and unenroll yourself.
In case of support / test / help courses, which are not listed in SITS, you can enroll on the courses directly in Moodle. Just select the appropriate department on the front page (or select it at the top, “departments” category) and you will be able to see all available courses. Alternatively you can use a direct link or you can use the search box at the bottom of the front page.
You will be asked whether or not you wish to enroll yourself for the course, this does not mean official SITS enrollment. If the system asks for an enrollment key (a password) it means that you can join only after the teacher shares the password with you.
For removing yourself from the course - In the administration block and navigation drawer you will see Unenroll me from "Course name". If you click on that, you will be removed from the course.
This only works for courses you self-enrolled, not for registered courses in SITS (usually you will lost your access for a Moodle course if you drop it in SITS).