Normally the name of the examination centre is printed on the hall ticket, if not you can either search for the exam board website, or ask at your school.
Often, details of where the exam is to be held are not known until a week or two before the exam so don't worry, everyone will have the same problem.
Your school teachers and admin assistants are there to help you, so please always feel you can go to them if you have a problem like this, there is no shame in not knowing, the only shame would be if you missed the exam because you did not ask where it was.
Study hard but take regular breaks to clear your head. I know this is a stressful time but it is so worthwhile and it will soon be over.
Just do your best, no one can ask more than that from you.