I think most of us are nervous. I was nervous too and it just takes some time to get used to the environment. Go to the test center early and try not to think too much before the test. relax and tell yourself that you are going to take it one question at a time.
A good thing about the GMAT is that it starts off with essays. So, it gives you time to settle down , get your thoughts together. After writing two essays, i am sure you will feel much better.
Of course, make sure you have practised with your essays. it is a good way to start off the quant if you had a feeling that you did well on your essays.
Hope this helps