There are two kinds of clothing in the world of Poser, Conforming and Dynamic.

Conforming is the most popular format, which involves creating the clothign model, then rigging it in Poser to fit and move with the figure.

Dynamic is less popular, and involves creating your cloth mesh, then running it through a simulation in the Cloth Room to fit it to your posed figure.

For both types you will need to be able to make your Cloth Mesh, for this you will need a 3D modelling program.  Some of these are rather expensive, others free, so it depends what you are willing to spend, if anything.

Blender is a popular and powerful free modeller, but it seems that the interface is a little strange.  Carrara 8 from DAZ 3D seems like a good purchase option, it has a good modelling tool set, and some integration with DAZ Studio, which in turn provides integration with Poser (in a roundabout sort of way).

My personal modeller of choice is 3D Coat, but that's a different animal altogether.

The first step then is to learn how to use a 3D Modelling program to make the cloth meshes for your clothes.  There are lots of modelling tutorials out there.

Once you have that I would go the dynamic route, because it is far simpler than the conforming style.  You will have your clothes up and running in no time.

Once you have that working you can go on to look at the Conforming style.

John

Fugazi (without the aid of a safety net)

https://www.facebook.com/Fugazi3D