Student assignment submissions appear in the Grades tab. From here you can view a student paper, make comments, and assign a grade.
(See also: SpeedGrader article, about this feature of the Canvas Gradebook.)
Opening a Student Submission
You'll also see notifications of student submissions on the Home page of your course.
Once you view an assignment, you'll often want to use SpeedGrader (see next article) to review the submission.