In their official Blog, Bethesda has announced that the RAGE tool kit is now available for download on Steam. This is the same tool kit that the developers' used to create RAGE and also it's add-on content, The Scorchers.
The tool kit includes the game's world creation tool called, idStudio, which can be used for creating maps, adding assets and everything else you need to get started modding the game. The game uses the idTech 5 engine so there should be plenty of eye candy features to keep modders busy for some time.
Look for the tool kit in the Tools section of Steam. The tool kit will only show up if you own the game, and is free for the taking. Happy modding !