When you log into your Teacher account for the first time, you will see a screen like this:
Simply click on the green "Create New Class" button and follow the prompts to label your new classroom:
After clicking "Create Classroom" you will see your new classroom displayed in the Classrooms list, with options to quickly get started with Create Assignment and View Assignments options:
At this point, you can continue creating all of your classrooms, or get to work with creating assignments!