First make sure you are already note enrolled in the appropriate course. If not, you should contact the Instructor of the course or their faculty administrator to make the request. An email will then go to Edtech or IT to make this change.
Please note that it is not possible for you to be both a student and a TA within the same course. Federal law [FERPA] and College policy both specifically prohibit students enrolled in a class from being able to modify their own grades or to view or modify the grades of other students enrolled in that same class.