Instructions

1

Choose a game to run. The most fundamental of things for a Game Master (or GM) to have is, of course, a game. Because running an online game like World of Warcraft requires you to be a company employee, it's just easier to attempt to run a tabletop RPG. Tabletop RPGs require game books, pencils, paper, dice and 2+ players. Oh yes, you also need a good deal of imagination, and some storytelling ability. If you're lacking any of this, no need to fret; it can all be found somewhere. We'll let you know where.
2

Familiarize yourself with the game. Most rules for tabletop RPGs are in book form. These can be found at large booksellers, game stores and direct from the creators (either online or through mail.) Once you've chosen your game, read up on it, immerse yourself in it. As a GM, you will need to be familiar with as much of the game as possible in order to accurately guide your players through the world.
3

Try some improv or take some classes. As a GM, you will be constantly required to think on your feet and come up with plot and dialogue off the top of your head. If you don't practice this or develop your improv skill, your games will come out as uninspired, lacking detail and boring. You don't want this!
4

Create a scenario. You've got to have something for your characters to do! Just plopping them down in the middle of somewhere and giving them no other direction will leave them confused and wandering! If you're not a storyteller by nature, at least come up with a list of characters and simple events for them to come across and react to. Things like "your house catches fire", "a hidden gunman takes a shot at you, narrowly missing your head" and "a rampaging motorcycle gang tears out of nowhere and begins riding circles around you" will give the players something to chew on for a while, but you'll need more. If you're a natural storyteller, write out a plot arc that will carry the characters through a series of challenging events. Are they in the middle of a war? Is their starship experiencing technical difficulties? Open-ended themes and events that allow for a number of possible reactions make for the best roleplaying.
5

Try and think of the people in the world around the players. These characters are called Non-Player Characters or NPCs. One of the keys to being a great GM is asking yourself "How would the NPCs react?", meaning how would the average people on the street react to what the players are doing? Keep asking yourself this, and I guarantee it will get progressively easier to be a Game Master.
6

Gather your players when you are sufficiently familiar with the game! When you feel you're ready, get your players together! RPGs require 2 players in addition to the GM (some say you only need 1, but most consider this type of roleplaying to be strange, obsessive and generally ill-advised.)
7

Try and keep your group manageable. Limits are different for everyone, but tabletop RPGs tend to get unmanageable at more than 5-6 players. Any more than that and side conversations begin to develop and it becomes difficult for the GM to keep track of where everyone is, what they're doing and the noise eventually becomes unbearable.
8

Lay it out. When describing the world and the things happening in it, try and be as vivid as possible. Does the forest have a thick mist that obscures nearly all vision, painting the landscape in a bizarre seascape of shadows that swim in the player's eyes and taunt them with visions of twisting limbs and jagged fingertips? Does the city belch smoke and fire, covering everything in it with a layer of ash and radioactive grime that never seems to wash off? Be descriptive, but remember that your players need to speak too!
9

Have fun! While action is the meat and bones of an RPG session, there's nothing wrong with interjecting a little comic relief. Perhaps one of the players botches a roll and falls down a manhole or walks into a flagpole? Things like that, properly described, can have your players in stitches!