Check with the history department to see if your college requires a separate application for a teaching assistantship or if the positions available are awarded with admission to graduate school.
Apply for the position, if a separate application is required. Be sure to list academic honors received as well as teaching experience. If your record is not strong overall, emphasize your grades in history.
Once you have received your assistantship, read all the secondary material assigned to undergraduates, including the recommended reading. Your students will probably catch you off guard from time to time: be as prepared as possible, but don't be afraid to admit you don't know something.
Develop discussion group lesson plans around the syllabus provided by the instructor. History teaching assistants typically take primary source texts and use the discussion groups to develop critical reading skills.
Take attendance and evaluate participation levels in every class. The grading system will be set by the lead instructor, but the teaching assistant has the responsibility of conducting most of the assessments.
Attend all planning meetings assigned by the lead instructor and follow his or her directives on lesson plans for discussion groups.