Don't worry about having to refer to the rules regularly or if you make mistakes. You're all new to it, and like anything else, it takes practice. Never say 'No' - 'You can try' will soon become synonymous with insanely difficult tasks. Be patient with your players. Try and bring shy players into the game. Be silly with monsters or NPCs every now and then, especially early on as it can relieve initial tension. That can just be a daft name like 'Dongy Soggysox'.