If you are a teacher, you can find the class code on your "Home" page to the right of your class name.
The class codes here are not visible so that you or your students do not inadvertently join another class.
If you are a student, your teacher will give you a class code to use. If your teacher is unavailable and you need your code quickly, you may be able to get your code from a classmate; everyone in the same class has the same class code.
If you are a student using the site on your own, you do not need to enter a class code. You can click "Practice" to do unlimited practice on any topic you want. Joining a class just lets your teacher give you assignments and quizzes and track your progress.