I failed my first one as well, for speeding... The nerves definitely played a part too, but for my second one I just tried to relax and rely on what I'd learned in the lessons. As someone mentioned already, you can chat with your examiner, which definitely helped me settle in at the start.
Good luck for your next test; I know a guy who took it four times before he passed and drives all the time now.
Final bit of advice, don't mistake a 30mph sign for 50...