The best thing is to take it as it is and be calm. You won't be any better than you are, but you need to trust yourself that you are good enough. If you are not calm, you could not show your full potential. In order to avoid such a scenario, you need to build your confidence, even if it is in artificial ways. If you have the time, watch this: https://www.youtube.com/watch?v=O6sE0tMcVwg . Whatever happens, be aware that you will learn from the experience and it will make you better.
Another thing is that no one can tell you "Be calm" and you can actually do it. Usually, this has the opposite effect. In order to do that, you need to hack yourself, in a way. You can do it by focusing on your breath and having a constant flow of air in, pause, out, pause. For this, watch this vid. You may also want the second part. It really helps. https://www.youtube.com/watch?v=q06YIWCR2Js