You can create a class in a couple of ways.
The first way is by clicking the large button with a "+" on it from the Teacher Dashboard landing page. Doing so will prompt you to name the class.
Simply enter a name and click the "Create" button and you're good to go!
You can also create a new class while currently viewing one of your classes. This is done by clicking the class dropdown arrow in the upper left, then selecting the "Add Class" option.