Well, first, it is possible to have a “bad trip.” However, that’s probably more about you being in a bad place, and it’s not the end of the world. You don’t have to worry about your brain chemistry, and you won’t go crazy.
If for some reason you feel yourself become anxious or nervous, be kind to yourself, like a close friend would be. Ask yourself why you are nervous. Ask those close to you, like your fellow tripsters, how you might change your mindset. This is frequently uncharted territory, so it is common to feel anxious when everything is unfamiliar. A good trip is merely a mindset away.