How to prepare for being a TA (Teaching Assistant)? I start my graduate program next week, which includes spending some of my time as a Teaching Assistant. It is my least favorite part of job, and I do not want to teach in my career after that. I don't want to become professor but get a job in a company.

Nobody told me how to teach and I don't even know what classes I have. I have little time to prepare, and probably don't know the professor (professors?) well. So what can I do to prepare, until I know what courses I have and can go over the material? I'm quite scared right now.
Be honest and be yourself. Students will latch on to your discomfort and will use it as an excuse for their performance. I have found that if you are honest with them with a statement along the lines of "I am new to teaching, so please be patient while I work out my teaching style and get more comfortable. If you have any concerns about how class is going, please speak to me as soon as possible." they are much more forgiving of your mistakes which you are bound to make. There is nothing worse than students who feel they cannot pass the class due to your teaching, so allowing them to voice their opinions early can both help in your teaching (i.e., "You stand in front of the board too often." or "It would be more helpful if you spent a little more time going over X.") and in your students attitudes towards you. But be careful on this front, make no instantaneous decisions. I always will say something like "I will think about it and get back to you next class if the policy X needs to change." Always get back to them about it, even if it is a no.

Additionally, talk to other grad students and faculty if you are finding you are having problems with teaching. I would be surprised if your department or university had people did not staffed just for training of instructors for teaching duties. Talk to them and voice your nervousness, they are there to help. Even think about all your classes and what the professor did that you hated or liked and try to emulate the best attributes as best you can. But don't try to be these people, just be yourself.

The key here is talk to everyone, do your best, and be yourself.

Since you're just starting out, I don't believe that they would throw you in cold into a classroom and say "teach" without any forewarning or even knowledge of which class, but they might depending on where you are (and talk to other people about what to do quickly if possible). You will probably start out grading and/or doing recitations/review sections a few times a week until you are more capable/comfortable with maintain your own class.

Good luck in teaching and studies! I always think being able to explain what you know to other people is a true sign that you know it yourself.

Addendum: The books that earthling suggested will always help. If you are in mathematics, I also suggest How to Teach Mathematics by Steven G. Krantz. Even though it is written for those teaching math, it is also a good read for almost any new instructor, particularly in the sciences.