It sounds like you're more into game design/level design than anything!
Unity is a good platform and it has a ton of tutorials/support if you're really looking to get into this sort of thing. Pygame isn't for you buddy, sorry! RPG Maker won't get you The Sims or Fallout either, and I don't personally think it will teach you a ton about creating games! It's more for storyboard writers who like drag-and-drop. Game Maker is a great piece of kit for those just starting out, but again you'll struggle to produce either of the games you mentioned ;)
There are plenty of free graphics websites - just google 'free 3d models'. Or even 'free game assets'.
If only creating the next Fallout/Sims was as simple as you make it sound - trust me, guys who have been at it for 10 years couldn't do that sort of thing on their own. Pick something simple - perhaps a run-and-gun game? You run around and shoot stuff.. Nothing complex there. Unity by default comes with a great example of something like this. Download it and give it a go :-) If this seems too daunting, stick with Game Maker for a bit and try something a bit more complex than apples and bananas... Good luck!