QFT. One of the best things for a designer i think is the ability to look at all sides of development, so that their view is rational. Knowing some programming languages goes a long way.
Also, you probably want to study LOTS of games. Even the crappy games have the ONE concept that made the publisher and developer think that the game would be a good idea.
I would also say read a lot. Maybe even a lot of history. A lot of good designers i have met originally majored in history, and a lot of the games that people think are new and inventive are old mythological stories or events of the past. It adds a sense of realism to every story.
If you more into just making the areas and building the levels, i agree with Schazzwozzer that building levels for NWN, NWN 2, or Oblivion is a good idea. But those are more for RPG's. You can build levels for Half-Life/Half-Life 2, Call of Duty, etc if you are into the FPS genre. Its good that you are young and are actively attempting to learn as much as possible.
*Note - this is stuff that i learned while working with the fine folks at Obsidian. My info is nowhere near as valuable as J.E. Sawyer or Chris Avellone's, but i like to believe that I've learned a thing or two!