We should put this in the FAQ.

Here's some links I posted in another thread.

Free modelling programs you can use:

http://www.planit3d.com/source/software_files/3dmod.htm

Making Poser conforming clothing with Anim8or:

http://www.morphography.uk.vu/modtut1.html

Making Poser conforming clothing with Wings3D:

http://poserpython.com/ClothTut1-1.html

And what Pakled said. Making clothing is not for beginners. If you're unfamiliar with 3D modelling, expect a long and steep learning curve.