It helps to be able to work with modeling software as well. 3ds MAX or Maya 3d were my favorites back in the day, but they're pricey. Mycosm is fantastic for creating semi-realistic worlds including atmospheric effects, physics, day/night simulation, as well as weather and seasons. If 3D Modeling and Animations sounds fun to tinker with, start out with the free but pretty robust Blender (for Windows, Mac, Linux, etc.)
As Bod pointed out, you'd have to know some coding as well. Also, you'll have to research game engines for whatever platform you'd be aiming at. Veraxon uses the Unity Engine. Some Engines are free for an X amount of end users, then start charging beyond that. Some are free, but crappy. Morfic would be able to fill people in better on what may be out there.