Did you ever dream as a kid that you'd stumble across a house made entirely of sweets and cake? You ate some of the door as you walked in, broke off bits of the table to shove in your bulging pockets, then you woke up and your wicked step mom told you that places like that didn't exist.
She was wrong! They do exist, and they are a lot bigger and better than you ever dreamt of as a child. Where is it? Down the phone line from your computer. It's the Internet. Just like the wicked witch in the story of Hansel and Gretel, the software companies that populate the Internet feel that giving away things for free is the only way to get customers to drop by. This actually encourages their competitors do the same, and in time the giveaways become bigger and better. A good example is Google SketchUp. Google decided that in order to increase the number of people worldwide creating 3D building assets for Google Earth, it would release the best asset creation software ever for free. Now the best marketing company ever is marketing the best asset modeling software ever and has linked it in to the biggest 3D environment ever which, by the way, it has also released for free. It's best to not overthink the possibilities, rather jump in and start using it. And that's what we'll do.
Google's 3D Warehouse is the place where anyone can upload a 3D model for others to download. It's like YouTube for 3D assets. It's worth familiarizing yourself with the 3D Warehouse right at the start because you'll find it an integral part of your game-level creation process. After all, you don't need to make everything yourself, especially if there are bags of good examples already out there. Next, looking at what other people have done well (and badly) helps you to hone your skills. Finally, you need to get an idea of what's popular and what's not if you are to sell your assets, and 3D Warehouse is a good place to do that research because you can see a lot of usage statistics.