A simple question to which the answer will, I suspect, vary pretty widely: how did you learn to GM?
For me, the short answer is, “From playing with Dave, David and Stephan.” The long answer follows — and the real answer comes at the end.
I got my start at age 10 with Avalon Hill’s Lords of Creation boxed set, which I bought on sale at a bookstore because it looked cool. I was fascinated by the equipment list (a fascination that continues today), and when I had friends over we’d each “buy” a certain amount of gear, and then I’d narrate some things that happened to both of us (playing ourselves as characters). Looking back, this was basically just an extension of “let’s pretend” games with the addition of a few loose rules.
On the bus to and from day camp that year, I did something similar with a friend whose name I’ve long since forgotten: more like storytelling with some interaction than actual gaming. Shortly thereafter, I met Dave, my first GM — and he introduced me to red box D&D, and actual rules. I couldn’t believe how cool it was, and playing it solo (one GM and one player) was a lot of fun.
I found out about a gaming store that was an easy bus ride away from my apartment, and went in search of what I’d played. They didn’t have D&D, for some reason — but they did have AD&D 2nd Edition, just released that year (1989). Books in hand, I started GMing for my best friend, Bud — again, one-on-one. I don’t remember much about the specifics of the game, though I know I didn’t use published adventures very often.
To get around not having more players, he played a host of other characters in addition to his main PC — and once in awhile, I’d take over one or more of them, and use them as NPCs as well. I tweaked the game quite a bit, often in pretty silly ways, and every time I learned something new or figured something out, I tested it on Bud (and that practice, to some extent, also continues today!).
From 12 to 16, I ran solo games for my friends. To this day, I have no idea why it never crossed my mind to get them together in once place and run something for them all at once, but I suspect it had to do with solo gaming still feeling pretty natural. More relevant to this post, I also played in a solo game with another of my friends, who was also named David (at one point, I was close friends with 7 Davids!).
David’s main game was set in the Forgotten Realms (still my favorite published setting), and he knew the world and the rules very well. We also sometimes played Marvel Superheroes, which I remember as being pretty madcap (I ran my whole superteam as PCs).
In high school, I ran a couple of short-lived games for two school friends, as well as a brief campaign for Bud and another friend at home. This pretty much stopped when I met my third GM, Stephan. Stephan was the GM for my whole group of gaming friends in high school, with others in the group (occasionally, myself included) taking turns once in awhile. He was (and still is) amazing at improv, and can happily — and successfully — run a game at the drop of a hat.
After another 4 years of gaming, I felt like I pretty much knew how to GM. In retrospect, I was wrong in some ways — but I knew enough to run some fun games in college, learn from the unfun ones and make some great friends in the process.
So…where’s the real answer to my question? The real answer is that I have no idea exactly how I learned to GM!
It wasn’t like learning to ride a bike, where there’s that one moment where you get it, and after that you can ride a bike, forever. And while I can pinpoint a few things — like the second Dave’s vibrant portrayal of the Realms, which was part of what led me to try and run immersive games — and the obvious bits like learning the rules, coming to understand how a GM could manage a group of players, and having fun gaming under good GMs, there’s so much more that I couldn’t identify if my life depended on it.
I would say that I considered myself to be a GM after my first couple of years running games (about 4 years after I first started gaming), and that from that point on I was no longer learning to GM, but instead improving on what I knew.
One thing I’m also quite sure of is that there was never a moment where one of my GMs sat me down and explained things to me. All of my learning came through playing in games, watching what other GMs did (not consciously, either — I didn’t keep notes, or set out to learn the craft) and trying things in my own games. “Organic” would be a pretty good one-word description of my learning process.
These days, it’s a bit clearer. I don’t think I’ll ever stop learning how to GM, and I’m much more active about trying to improve my craft than I was before I knew that there was a craft to improve! I still learn a lot from playing in games, watching what other GMs do and trying things in my own games — but now there are other resources as well: messageboards like EN World  and The Forge , not to mention sites full of advice, like Roleplaying Tips , and books all about GMing (Robin’s Laws of Good Gamemastering, etc.). I’m guessing that if I started learning to GM now, some things would go quite differently — but others would probably stay the same, and I think the organic aspect of the process would be very likely to remain.
That’s my experience. What was yours?
Edit: I posted this same question over on EN World and The Forge (my first post on the latter!). If you’re curious to see how others answered, check out this thread on EN World  and this thread on The Forge . Interestingly, I was more or less completely wrong that answers to this question would vary widely: by and large, most posters in these threads had quite similar learning experiences. I think that’s ripe ground for a future TT post.
(The seed for this post was planted when I read a post elsewhere that talked about the idea of having classes for GMs — an organized learning process, in other words. For the life of me, I can’t find that post, and I have no idea where I saw it. If it was yours, pipe up in the comments!)