Start with a pre-defined adventure
The role of Game Master is full of responsibilities. This load can be lightened by using a pre-defined adventure. Many may balk at this statement, saying that this hampers creativity and GM freedom and it very well may. The main reason I advocate for this path is that being a good GM takes practice. Practice is typically best approached by starting with simple steps to build confidence and then adding more steps that grow in difficulty. This is what a pre-defined adventure is good for. Think of it as training wheels.
Using a pre-defined adventure frees the new GM to spend more prep time on learning and understanding the rules instead of working on adventure design. In addition, the time spent on rules can be more focused. By looking at the elements of a pre-defined adventure the GM can focus study on just those rules that will come to play. If none of the planned encounters deal with social combat then no time need be spent on that subject. Also, by careful study of the planned adventure the GM can be ready for possible deviations by the players and therefore seem more knowledgeable and competent regarding the setting and/or game system.
Also, by playing pre-defined adventures the GM will learn more about ways to use the game system to its fullest. The presumption here is that these pre-defined adventures are written by authors more familiar with the setting/game system who will be able to ecourage a richer game experience. As time goes on the new GM will be able to define which elements of the setting/game system they enjoy and which ones they and their players dislike. With several sessions under their belt, the GM may begin modifying existing adventures to better suit the needs of the play group, evenutally leading to solid confidence necessary to design original adventures.
Communication is key
It is important as a new GM to not assume anything. This is esspecially true when discribing situations and scenery. Having read the adventure and the rules several times they no doubt have a clear picture of the environment and situations. It is importnat to convey that clearly and effectively to the players so that all may participate in what Ron Edwards call the Shared Imagined Space (SIS).
In an example from our actual play, the adventing parting started in Sharn on the world of Eberron. The GM had a clear picture of Sharn from his readings as did I from previous play experience. However, my younger son had no experience whatsoever with Sharn. He had no idea what is was like and as such could not participate in the SIS. I pointed this out during play by asking the GM to discribe the city since all the characters were first-time visitors. This helpped bring everyone at the table together.
It is also important to clearly listen to what the players are telling you as a GM. Ask clarifying questions that help define further the intent of the player. If the player simply says, "I attack it!" when there are several targets, clarify by asking which target so there is no mistake later, "No, I was attacking the wounded one!" A simple question goes a long way to avoiding such difficult situations.
GMs should be careful that their questions are not leading. "Wouldn't you rather hit the wounded one?" verges on taking total control of the player's character. The players will learn from experience just as the GM does; don't rob them of that opportunity to learn.
+1 Swords are typically not labled as such
This may be a tough one now-a-days. Within the realm of electronic RPGs (both on-line and console versions) a player typically knows exactly whether or not any items gained are magicial as well as the exact nature of the magical effects. However, in a tabletop RPG this need not be the case. The determination of magical items and their respective effects give players a reason to cast Detect Magic and Identify spells as well as encouraging testing a character's arcane knowledge. This is part of the mystery and wonder in the game. Also, it gives players a reason to spend some of that coin they just worked so hard for. If characters have plenty of cash there is less of a desire to continue adventuring, while one strapped for cash will be eager to venture forth to raise cash for next month's lodging.
That being said, new GMs brought up on electronic RPGs may not see the reason to have the players "jump through hoops" to find out the sword is only +1. My point for bringing it up here is that new GMs should consider their approach to the situation before blindly falling on habit.
Don't be affraid to make mistakes
Everyone makes mistakes when starting out. I've already talked about one my biggest here when I first started GMing. It is inevitable; there is a rule that you thought you understood, but once in play it no longer made sense to you. Keep the ball rolling. Don't stop and fixate on the mistake; pick up the ball and keep running. Chances are the players won't even notice (unless, of course, they're the ones that pointed it out to you) and you can keep on going.
After the game, take time to relect on the session. You may even ask your players: "What went well? What didn't work so well?" From this reflection you will discover where to focus your attention when preparing for your next session. In time, all the tasks and responsibilities of being a GM will become second nature to you. That's when you're on your way to becoming what Gary Gygax called a Master GM.
Follow Your Bliss,