I recommend looking at Google Sketchup. It's a 3D modelling program that's mostly aimed at architectural modelling; it lends itself well to starting with a layout or side projection and extruding from that. It's got a bunch of tutorials and can export in several useful ways.
However, Sketchup will work best if you're designing 3D environments, not things like humanoid models. It doesn't really do the same sort of vertex-and-polygon-based archetype as typical 3D modellers, and I don't believe it has any sort of skeleton or animation functionality at all.
If you're feeling adventurous, I recommend trying Blender, like some responders have suggested, since it's free. Look for tutorials online. It will take a lot of learning, more than Sketchup, but it's a fully-featured modelling program.