In the grand scheme of things, your role as a TA is three-fold:
- Assist in instruction for math classes. This department would not be able to function without the logistical (and, frankly, inspirational) support of TAs like you.
- Develop teaching skills that you'll need when you graduate from UCSD and look for a job.
- Actively participate with students, instructors, administrators, graders, and staff to improve the UCSD experience. This campus is a community, and you're part of it.
Hopefully this is most of the logistical support you'll need. As far as the actual teaching is concerned, please see here.