Contact the history department of your college or university to see how you can become a TA. Some schools require you to complete a separate application, while others award TA positions to all of those admitted to the history department's graduate school.
Complete the TA application, if you have to apply for specific positions. Emphasize any teaching experience you have, your academic experience with the course you are applying to be a TA for and your overall academic standing.
Speak with the professor of the history course if you are hired as the TA for her class. Ask her for the class syllabus, a list of the readings for students and what the expectations she has of you -- including your marking workload and your assigned classes.
Read all of the material that has been assigned to the students before they do. You want to be as prepared as possible if and when students come to you for guidance.
Attend as many of the history classes as possible. While some professors do not ask that you be at the majority of classes, you should try to stay up-to-date on what is being covered in each class.
Hold office hours for the students of the history class at reasonable times. If you are a TA of a small class, you might ask everyone what times would work best for them to hold office hours. If you are a TA of a large class, you probably won't be able to accommodate everyone's schedule perfectly; in these situations, spread your hours out throughout the week and give the students your email address so that they can write to you if they can't attend office hours.