When your teacher assigns you homework, you will receive notifications in the Magnet bot conversation in the message center, in the notifications carousel on the Dashboard and in the notifications carousel in the corresponding group.
You can access the assignment directly by clicking on the link on the message (in the message center) or on the "Start" button (in the notifications carousel). You can also check if have any new assignments in the "Assignments" tab of your group.
Once inside the assignment, you must click on "Start" to begin. Depending on the type of activity assigned, you may be able to get instant feedback. Otherwise, you will have to wait for your teacher to correct the task.