How to prepare for being a TA (Teaching Assistant)?

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.

You are off to the right start - ask questions. Next step is read... a lot.

I have some books I've recommended here before. They will give you a good beginning.

Sadly, university teachers are rarely prized for their teaching abilities. Often they are just thrown in as long as they understand the subject(s). Tertiary teaching is, more often than not, learned on the job. So, read all the posts here about teaching. If you have a chance, practice. Usually a university teacher's first semester is the worst. It gets much better (and easier) as you go on.

One other course I would recommend is The Art of Teaching. It is really well done and covers so very many things you should be thinking about.

Accept that the first day, and probably a few days following that, are likely to feel a bit 'emotional'. It happens to everyone. It will get better from there quite quickly, though will probably take at least a term before you really get into the swing of things.

If you haven't done so yet, try going to the admin department and seeing if they know what courses you'll be doing. They're useful people to know in general, and I've found them often to be much more on top of things like this than the faculty. The same applies to lab technicians and other staff (who will absolutely know a heck of a lot more about this stuff than you do, so stay on their good side).

If you're not already, get familiar with the way things are generally 'done' at your uni. With undergrad courses and practicals there's quite possibly a standard way all work is assigned and graded. Look in the undergraduate prospectus and/or any webpages set up for them to get an idea what they're being told, and hence their expectations. Also look up any rules your department has on plagiarism, dealing with late work, dealing with sick days and religious holidays, what constitutes appropriate and inappropriate behaviour towards them on your part, etc.

You will almost certainly need a calendar to keep track of when you're meant to be doing what. I've used a pocket book in the past, but these days would probably use my phone so I can sync it all online. Being organised, and knowing that you're organised, removes a lot of the stress.

Once you find out what course you're doing be sure to go through all of it. If there's a practical demonstration, do it yourself as soon as possible - and once you can do it, play with the equipment and find all the ways it can fail to work (your students will discover them all). If there's questions students will be expected to do, do them all yourself, no matter how easy they look - you need to know this material better than they do, so you can deal with the thousand and one bizarre errors they will make. This will be a bit of a time sink, but you've got to do it.

Be prepared to not be surprised if you start to find you're enjoying it. Even if teaching isn't your long term goal, seeing someone 'get' a problem thanks to your advice can be very rewarding.