There are several ways that you can ensure that you join and consistently log into your instructor's course.
Know your course ID!
Your instructor should give you your course URL, which contains your course ID. Make sure you purchase or register for your course from this URL. Also, after you've gained access, you should log in from the same URL.
Locate the Course and Instructor's Name on the log in screen.
You can find this information by looking on the log in screen, on the left side of the page, below the LaunchPad title.