It does seem so daunting and impossible, but like everything else in the grad school experience, you just figure out how to do it. I assumed I would have a mild TA assignment in my first year, but when I arrived on campus I found out I was to be a lab instructor (meaning I had to teach two sections of an 80 minute lab/discussion for a Research Methods course) for a 300 level course. It was a crazy work load because I had to prepare my lectures and have office hours and do tons of grading. But like I said you just figure it out and you do it. Hopefully your school will have some resources to train first time teachers. At my university this was a one day seminar for teaching fellows. Your advisor and other grad students should be able to give you some tips and support as well. And don't forget, you're not the only one. Everyone else in your cohort is just as freaked out as you are. Good luck!