I was out at breakfast with a friend the other day, talking about my current board game projects, and he asked me how I got into game design in the first place. Here's the story:
I played the boxed set of D&D and then AD&D, starting around age 7 - way back in 1977. Soon I started making dungeons, and like everyone else, pored over the various books and manuals for the game. One part I really appreciated was the note from Gary Gygax at the beginning where he said that the book was a guide and that you were free to use or not use whatever section or bits you like, or even invent your own. What a great thing to hear as a young person! This was a game that included your creativity and imagination. I took it to heart, and when playing any game, I always wondered what I might do differently if I made the game. So that was a good perspective to start from.
The other thing that helped was that my father got a Commodore PET, a (now) ancient computer that had a cassette tape for reading in programs. We got some cassette tapes with many programs on them, mainly games. Everything was in basic, and you could see the code. And even better, modify it! So my brother and I, even before we knew programming, learned how to delete a line or two. We would delete different lines of code and then run the program and see what happened. In one game, Dungeon - a simple kill the monsters and gather treasure dungeon crawler, we found the perfect line to delete in order to change the shape of the dungeon. Instead of rooms and corridors, it made a random huge room with pillars! How fun!
After that I learned to program BASIC, and was always trying to make adventure games like Colossal Cave. Find the rust remover in a random field and use it on the stuck lever so you can deactivate the electric fence and go somewhere. That was in a game I made when I was 10 years old or so.
From there is was making and playing a lot of games. I am a huge fan of iteration for learning how to do something, and how to refine your designs.