If you have been asked to join a class by a teacher, select their name from the drop down list. Then select the class they have asked you to join. You will be asked if the details you have selected are correct. If they are not, select No and reselect the correct fields. If the details are correct select Yes and you will be automatically added to the class list and will start receiving zApps and zApp Books assigned by the teacher.
If you have any questions or if a teacher or class is not appearing in the list then please ask your teacher for help.