How do grad schools train grad students to be teaching assistants? I am an international student and doing Ph.D. in Physics at a US university.

My teaching assistant (TA) duty is to tutor undergrad students. I am supposed to solve whatever problems undergrads bringing to me. The problem is that I do not know which problems they are having to prepare in advance. Every undergrad from every lab, class in the physics department can bring whatever homework they have to me to ask. Also, I am an International Student and when I was an undergrad, I did not use the textbook undergrad here in US use and many problems are totally strange to me. Sometimes I cannot solve their homework problems. I felt very embarrassed and sorry since I wasted their time, sitting there for 15 -20 minutes to wait for me to solve it. 

I am not a terribly bad student, I consistently perform about 80 percent for all the courses as well as standard exams like GRE Physics. But I feel like I am not smart enough to pursue a Ph.D. Sometimes I can come up with very good solutions for grad problems but I am not a fast thinker to solve some undergrad problems which might be solved in a very simple way.

Do grad schools train grad students to be teaching assistants? I think the Ph.D. students need to be prepared to do good TA jobs, isn't it? For example, if someone teaches labs, they should know the content of the lab for that day to come in preparation? How can I become a better TA given my described task?
How do grad schools train teaching assistants?
Some graduate programs offer specific workshops to teach the fundamentals of teaching. However, being a good TA usually comes down to 3 things:


Proper understanding of the material
Experience in teaching beforehand (I am a much better TA now compared to a few years ago)
Preparation (this is necessary even if you are experienced)


Nonetheless, the content of your actual question (and corresponding comments) is quite worrisome. It is absolutely not the responsibility of a TA to be prepared to answer any question from students of every lab, especially without previous preparation. I would dare to say this is not even the responsibility of the professor teaching the course. I have yet to meet a professor who doesn't feel the need for previous preparation when teaching a course for the tenth time, nevermind teaching new courses for the first time.