One of my groups asked about what advice we would give new GMs. Here's what I said.
"Don't be a dick. Power comes with responsibility. Know the rules but know what your players can do. Research the world or devise your own. Give players a heads-up on any houserules before the game starts. Be consistent always. Use voices. Be silly. Ham it up. Rules are important but not at the expense of the story. When I talk about rules I am never talking about using them against the players. You can and will be challenged. You will be part psychologist part bartender and part storyteller. You are NOT writing a novel by yourself. You are in an immersive (that is a real word Facebook) shared experience. You will at times get mad at your players. It is ok to be challenging it is NOT ok to 'teach them a lesson'. Your monsters, encounters, etc will ALWAYS be able to destroy their characters. Don't do it. Be fair. Be honest. Be fun. Show them a good time and you'll have a good time too. Rest of advice I leave to years of game master experience."