This may help with creating clothing within Poser.
http://www.philc.net/BasicClothing_page1.php
The aim of the scripts is to enable the novice user to make quite acceptable simple clothing in a very short time without a vast degree of skill.

I would also suggest that you get and become familiar with a 3D modeling application that you can use to either adapt existing models (as made by the above for example), or create your own from scratch.

Be prepared for a learning curve, it is not easy, at least I did not find it so. However if I can do it then I'm thinking pretty much anyone else can too :)

My current 3D modeling application of choice is Blender.
http://www.blender.org 

Hope that helps.