I agree with Hades, NVN is a nice editor to try learning with since it since it is based off of a modular building system, and the event scripting for it is fairly easy.
Morrowind and Oblivion have downloadable editors. If you want to learn how to edit with either of these check out www.elderscrolls.com and check the forums for editor and mod help.
Then there are the FPS games with editors, many of which liscence out their engines to game companies to use to make games with. Some of the most prominent are Doom 3 or Quake 4, Unreal Tournament 2004, and Half-Life 2.
There are large modding communities for pretty much any game that can be edited.
Asides from editors, I would suggest starting to learn other skill sets that are part of making video games.
Image editing software such as Adobe Photoshop, 3D modelling and animation software such as 3ds Max. Scripting languages such as C++, Java, etc.
Things that you could take in school that I see as beneficial are classes such as creative writing and art classes, however the more you know about as many things as possible, the better, since all knowledge enriches you and gives you a greater resource pool to pull from when designing games.
