To find a student's username, go to your Manage Classes page and click on the name of the class the student is enrolled in. You will then be shown a roster of your students' names and usernames. You can also print out a copy of the roster by clicking on the Print Class Roster button in the upper right hand corner.
If your student has forgotten their password, please see the FAQ: My student has forgotten their password. How do I reset their password? Can students reset their own passwords?