My advice would be to start out with dynamic cloth.  This will be a less steep learning curve than Conforming Cloth.

You will of course need to learn to model clothes in a 3D modeller, as Robyn said, but once you have modelled the garment it takes far fewer steps to get your garment working in Poser.

In my experience the modelling part is the simplest part of making Conforming Cloth.  the rigging and setup to get it into Poser takes twice as long :)

With the Dynamic method you can concertrate on your modelling skills for a while, whilst still getting usable results, before getting into the joys of Conforming Cloth.

If you do not already have a modelling program I would suggest getting hold of a copy of the latest 3D World magazine, which has a free copy of Carrara 7 on the cover disk.  It's a good program, and for free is great :)  I use 3D Coat to get my basic mesh, then Carrara to add details.  Have a look in the Video Tutorials on rendo, I have a few videos on there which show how to make a dress in 3D Coat.  My videos on YouTube (looks for Fugazi3D) have lots of information about using 3D Coat and the Cloth Room in Poser.

John

Fugazi (without the aid of a safety net)

https://www.facebook.com/Fugazi3D