In Japan, a personal seal called a hanko (判子) is used instead of your signature on most documents. People often use hanko and inkan (印鑑) interchangeably, but technically speaking, your hanko is the cylindrical object made of wood, plastic, etc. used to make your seal, and your inkan is the red seal that you make on paper using your hanko. Since a hanko is necessary to complete all of the important documents you need to fill out during the first week or so you are here, you should take care of this as soon as possible. Your supervisor will probably take you to get one, but if nobody brings it up, make sure to ask about it. Your supervisor may suggest that you get the seal made in katakana, but you are welcome to get it in kanji if you like. Ask your fellow teachers or Japanese friends to help you find some good kanji to use.
You will be using the Hanko on almost a daily basis. On the days when you you are required to work you will need to stamp your attendance in the staff book.
You may also consider getting two hankos, one for home and one for work. Some schools can be pretty strict with how many hankos you can have, or even where you allowed to keep your hanko (i.e some schools prefer the office ladies to hold onto your hanko) so make sure to ask your supervisor if you are allowed to have several hankos.