At some point today, the RAGE tool kit will be released to Steam for your user-content creating pleasure. The download will be pretty dang hefty, weighing in at over 35 GB.
Bethesda made the announcement earlier today on the company's official blog. The kit utilizes "the same tools used to create RAGE and its DLC, The Scorchers." Packed with assets and game maps, users will presumably have all they need to start modding the crap out of the first idTech 5 title.
Before getting started, Bethesda does recommend checking out some reading material they've provided. The tools are understandably complex and targeted towards "technically sophisticated and adventurous" modders. Those documents can be found here, on Bethesda's blog.
When the kit is released, you'll be able to find it under the Tools section of Steam, the same place where you may download Skyrim's Creation Kit.