Studies come first. It is a given that most GTAs must earn As with only an occasional B. Attending all classes, being on time and coming to class prepared are also expected. Your reputation as a student matters as much as your performance.
Most GTAs will teach. This usually involves being completely responsible for creating and implementing the syllabus, using the teaching strategies learned in coursework and applying the theories you've aligned yourself with. Also involved are assessing students' work, documenting grades and holding office hours.
Papers will be scrutinized for originality of scholarship and the effective and accurate use and documentation of sources. It is expected that you are already familiar with the current theories and prominent theorists in your discipline, even those not discussed in any of your classes.
Some schools consider GTAs junior colleagues, inviting them to participate in committees such as textbook selection. Teaching assistants write and publish articles and attend and present research at conferences. They support professors by performing administrative and research assistance and show professionalism by speaking positively about professors to students.
Technology is a part of modern classroom teaching, and many schools require instructors to use software such as Desire to Learn, WebCt to document grades and post supplemental reading material and announcements. Become comfortable using available technology to enhance student learning.
Teaching assistants must be excellent time managers to effectively carry out the roles expected of them, especially because they will be expected to produce quality work despite family, personal and time demands.