I haven’t ever played in a tabletop RPG, much less GM’d, DM’d or refereed one. I’ve got the itch to try it out, though, so I’ve been compiling articles and tools about learning how to GM. Here’s a list of what I’ve found. Much of the material written on this topic is specifically about Dungeons & Dragons, but the advice can be applied more generally; myself, I’d really like to try running Star Wars Roleplaying Game: SAGA Edition or maybe Spirit of the Century. Here’s hoping these resources will help me (and maybe someone else) get started!
So You Wanna Be A DM? by AKA_Bait is the most friendly and thorough introduction to the topic I’ve found.
How to Run Roleplaying Games by Greg Stolze. Very professional.
A Quick Primer for Old School Gaming by Matthew Finch talks to both players and GMs about an older style of RPG… but to me it just sounds like good principles for running a fun game.
Rama’s Rules of Running an RPG by Jason Rama is a quick list of things to remember when learning to GM.
GM Guide by Colin G Hetherington seems like a very detailed resource, but it’s incomplete.
17 Steps to GMing a New RPG for the First Time by Martin Ralya is about your first encounter with a new system.
How to GM, DM, ST or just Run an RPG by lhewitt is a shorter and more general introduction to the topic.
The Rancor Pit – Gamemaster Tips for the West End Games D6 Star Wars RPG.
The Tao of D&D is a blog about making the rules serve the game, not the other way around.
Newbie DM.com has some interesting resources.
Gnome Stew claims to be the #1 GM blog on the planet.
Roleplaying Tips posts regular articles for DMs.
Spirit of the Centry can be played for free from the SRD on their website.
Swords and Wizardry is a totally free, very simple game that’s a throwback to the earliest fantasy gaming. Lots of resources here.
Castles & Crusades Quick Start Rules are also free classic fantasy gaming.
Savage Worlds provides a Test Drive to try-before-you-buy.
The Four Color System is a free superhero RPG.
Have-A-Go Heroes is an extremely simple and quick Superhero RPG based on Mystery Men.
The West End Games D6 System core books are free PDFs.
The Dawn of Defiance Campaign for the SWRPG SAGA Edition would be a good thing to DM first. Not sure how badly you need the core book for this.
True20 Quickstart Rules for yet another fantasy system.
Blue Rose Fast Play is a free-to-try adventure.
GURPS Lite is a free condensation of the commercial GURPS roleplaying system.
The Order of the Stick is a D&D Webcomic. It never hurts to see how others have had fun with the genre/game!