Everyone has an allowance per year on what they can earn before they are taxed. I think it is around �5000. You may be "emergency taxed" when you start working, but this will all even itself out and you can claim it back. It is important that you make sure you pay NI, as it could come back and bite you on the bum eg- your pension / hospital and doctor stuff we take for granted as free.
Best thing to do is to phone your local tax office. Despite the name, they are usually very nice and helpful, and appreciate you asking questions.