To help you understand these issues more clearly, let me add that US copyright law (and it's a good point that was brought up that this is ONLY in the US and other countries that reciprocate) allows for what is known as "Fair Use" for education and satire.

So if you wanted to make a Mario model to learn modeling techniques, that would be fine. To learn how to integrate that model into a game would be okay as well if you don't release the game. But doing it to make a modle or game that you will make public, be it for free or not, would not be okay.

Likewise, if you wanted to make a satire of Nintendo or the Mario franchise, it would be okay too. However, note that this is iffy for one mans satire is another mans sequel if you catch my drift.

So long story short, as long as you keep your works private and are only using them for educational purposes (so for example you post screenshots of your work for critique) but never release them, you would be fine under the "fair use" clause. It's anytime you make things public that Copyright law gets dicey.