If you have not yet added a webinar session to your course, you can take a look at this article to see how to add one.
Once you've scheduled one, you have two ways of beginning the webinar: from the Dashboard and from the Course Preview.
If your webinar is scheduled to begin at a future time, it will appear in the Upcoming Sessions section. You can click the blue eye button to launch the webinar.
If your webinar date/time has passed, you can still launch the webinar by navigating to the course and hitting the Launch Course Preview button.
From there, click on the webinar that you have added.
Both methods will take you to a page where you can Launch the webinar, find the Attendance Key which learners can use to mark themselves present by entering it, as well as a list of registered users with a button that you can use to Mark as Present.
Once you launch the session, you will enter the BigBlueButton webinar interface.
While using the webinar tool is relatively straight forward, BigBlueButton has a wealth of tutorials that can be viewed here.