I'm a cover supervisor in secondary school.
Wedgiesmum is a TA in primary school.
Nikkie BSL interpreter/TA in SEN school (I think - hope I've got your job title approximately right Nikkie!)
Whizz, you may well find that at times you have duties to support/supervise children at break times and lunchtimes. When you are free, do make sure you socialise with the whole staff in the staff room/refectory/wherever. Even when you are really busy (and you will be), make time to grab a coffee in the "place to be". In some schools TAs tend to cluster together in a ghetto, and then complain when they don't feel integrated into the whole staff.
When on classroom support, if you are not sure what you can do to support the teacher then do ask. Particularly if you are supporting regular classes, then grab a quick chat with the teacher at a breaktime, and ask for feedback on your performance so far, and ask what you could do to help and/or should not do. We have some fantastic TAs in our school, and the odd occasional chocolate teapot.
If you are supposed to be supporting in a class and will not be there for whatever reason (sickness, absence, training, or if your Manager has told you to be elsewhere), then make sure and double check that the teacher is also informed. If they can rely on you being there, you will be of far more use to them, and they will plan to use you most effectively.
If you are supporting one-to-one in a classroom, and expect to take that child out of the class (for a reading test or something), try and give the teacher advance warning, and ask if it is convenient.
Most schools rely heavily on email for communication, but most TAs do not get given laptops. Do make sure you get a username and email address asap (you may have to push IT for this, as they are always busy); check to make sure you are included on "all staff" emails, and try and find regular time to check your emails. This will help you feel more informed about the school, and more a part of the whole structure.
Hope those few "off the top of my head" comments are helpful to you!