First of all, welcome to the community. To create clothing for Poser characters you will need a 3D modelling program such as 3D studio Max, Rhino or Maya or even one of the freely downloadable ones like "Wings 3D". Model the clothes as a mesh and import them into Poser. From there you can make your clothing Conforming or Dynamic. I suggest you read the tutorials available on Making conforming clothes and start from there.