Exams are known to be very stressful things, but by learning to cope with them you can reduce the stress on yourself and have a better experience than if you don’t learn to cope and struggle your way through.
You should be issued in advance not only your timetable for your exams, with dates and times clearly shown, but also some exam rules, which you need to read thoroughly and make sure you understand. By now the school should have subjected you to exam practices; whether it was your CAT tests, or some other form of tests, they will have insisted that you do these under full exam conditions, and this is intentional. It means that the GCSE exam hall will not be a surprise but will be something you are familiar with, thus hopefully reducing that stress.
Make sure you comply with the rules, hand in your mobile phone, or leave it at home, have your clear pencil case or plastic bag with the required equipment in it, have clean tissues, especially if you suffer during the hay fever season, and be at the hall on time. If you do suffer from hay fever the school may be able to make provision for you to sit as far as possible from a window, or may allow you to sit in a small room that has no open windows with others who also suffer. Use the toilet before you go into the hall as this is less disruptive.