If you are not a modeller (and I am not) I have found two options:

1. Philc has a clothing designer package on his page at philc.net. It does not require modeling skills. About $60.

2. Pegasus, available over at Daz, is a self-contained package for creating clothing for Poser people. About $40.

I own both, and have found that you can get there using both. PhilC's package is simpler, but Pegasus gives more flexibility IMHO.