Everybody gets nervous before exams, don't worry. I actually used to throw up every morning before an exam simply due to nerves.
My main advice would be to train yourself to become more used to it. It might help to take past papers at home in complete exam conditions as often as possible. This way you're revising at the same time as learning to cope with the pressure of taking the exam, which will hopefully help with your nerves. Plus, by getting used to the exam format you'll avoid any panic that could occur in the actual exam as you'll know exactly how the paper is structured.
Something else that really helped me was to picture myself succeeding. I know it sounds really cringey and cliche but imagining myself acing the exam I was about to sit and telling myself that I really could genuinely helped me with my nerves.