It depends on the age of the children you would be teaching but what you would usually say is:
My name is Mr/Miss/Mrs and I'm going to be your teacher in (Class Name).
Then state a few general rules you have with children at their level,e.g. when I'm talking to a class of 6-7 year olds I normally say "I want you all to try your best, listen when I am talking and try to be nice to others".
Make sure you tell them how pleased you'll be with them if they follow your rules and that you hope you'll learn lots of new things and have a lot of fun learning them.