For teacher accounts, you will first need to setup a class. After you have done that you can add students in two ways:
1) Use your invite link to allow your students to add themselves to your class. This allows your students to pick their own username/password and eliminates the setup work from your plate.
2) Or you can add a student to your class by clicking the "Add Student to [class]" button. You will see a popup window that will allow you to enter information for your student. You will then need to send your student the username/password that you setup for them.