If you are a new student, it is likely that you did not have any choice in selecting the course that you must TA. There isn't much you can do to change that. If you are unhappy with your assignment, just try to do the best you can. If you are unfamiliar with the subject matter, it will be more of a struggle, but you'll survive and you will have learned something new. You should have more say in what courses you TA in subsequent quarters.
This year Prof. Carl Ebeling is in charge of TA assignments. If you are unhappy with your assignment, or have a particular course preference, you should talk to him. If you have a particular expertise, notify him of this as well; it may help your chances of being assigned to the course you want. If you are interested in a particular course, talk to the professor; professors may request particular TAs for a course. You may also try to trade positions with other students. Consult with Prof. Ebeling regarding any agreements you make with other students.
There is an ESL (English as a Second Language) test required for foreign students before getting a TA. Consult the Mentor handbook for more information.