First, you'll need to sign in at lagunita.stanford.edu/login. Make sure to enter the email address and password that you used when you first registered your account. If you have not activated your account yet, you will see a message telling you that you need to do this before entering a course.
Once you have signed in, you will land on your Dashboard page at lagunita.stanford.edu/dashboard. This page shows all of the courses you have registered for (if you haven't enrolled in any courses yet, you can learn how to do that here). Click the "View Course" button to enter your course.
If your class has not started yet, a "View Course" button may not be visible until the start date. Courses which have ended will display "View Archived Course" buttons.