Blender software, http://www.blender.org/
it works similar to 3DS Max and Maya (which I use for class projects) but it's completely free. I don't really use Blender myself, as its interface and controls are very different then the Autodesk software I am using.
Also, there are a lot of Blender tutorial video's on youtube as well as various other sites, you just need to google them.
But again Blender is almost as good as 3DS Max and Maya.
....by the way don't expect to just know how to make something look realistic right off the bat. Heck don't expect to know how to use the software, period, right of the bat, it isn't going to happen. I've been learning and using 3DS Max for two years and Maya for about four months and I am just know at the point were a tooth looks like a realistically detailed tooth or a hand looks a hand and not some deformed creature.
Unless your a gifted prodigy (or something like that), you'll need to dedicate much more then a week or two just to know and remember where all the tools are located and how to use them correctly. Because there are many dozens of tools for various things. Then several weeks, even a couple months to learn how to make and manipulate shapes and objects correctly and make cool things.
I hope I haven't discouraged you, because being to use 3D modeling software is a lot of fun and extremely rewarding!!! But learning how to use it can be rather frustrating!! ;-)