To be a successful TA in the United States, you must understand the expected roles of students and teachers. You may have to learn new ways of teaching, ways that are now unfamiliar to you. In American higher education, students are often asked to evaluate their teachers. Negative student evaluations may mean that your teaching assistantship will not be renewed. The job of a teaching assistant is not an easy one.
Some departments invest more than others in developing the teaching skills of their TAs.
Some universities do more than others in this regard. To get the most out of your situation, consult with experienced TAs on your campus, and learn exactly what is expected of you.