If your school or district doesn't set student usernames and passwords centrally, you can set them yourself for your students.
Click on your email address in the top right of your dashboard. In the dropdown menu at the top right, click "Student Passwords":
You can then download a CSV of your class. Edit the username and password fields only. Then upload the modified CSV.
Usernames and passwords aren't required for students. Students can also log in by typing your email address in the username field and selecting their name from a dropdown. (Note: some districts/schools may have opted to disable the dropdown option -- in these cases, students can only login using usernames and passwords.)