I get really bad nerves. Like, so bad that I often get physically ill. I couldn't sleep all night before my driving exam. I tried buying some of those drops that are supposed to help relax you but I don't think they did much. Anyway, this sounds really really simple, but my best piece of advice is to make sure you remember to breathe! I kept taking really deep breaths that practically sounded like big sighs but everytime I did I could feel some of the nerves go. The examiner even asked me if I was okay and I said I was just really nervous. Anyway, I swear it does help if you take big deep breaths.
Good luck! I failed my first time around. It's pretty common. I think the national pass rate is something like 44% (and that includes retakes) so you're not alone! At least the second time around you'll know what to expect.