So far, I've only learned one way. The Basic Human Tutorial in the freebie section by Kurt will show you how to make the clothes from a modeled polygon character. Then, by the guidance of another member, converted it to nCloth. I just selected the polygon shirt and then under the nCloth menu, clicked "Create nCloth". I did the same for the shorts, worked great.

You can also create them from curves but someone else will have to tell you how. In the meantime, try to research it with your help files (F1).