What program? There are a lot out there so you have to be a bit more specific.
As far as my limited knowledge of skinning and theming goes, I work mostly with PhotoShop and
http://astonshell.com. If you want to use that all you have to do is download and install the 30 day trial. And if you want to make a theme but don't really know how to get started, go to my website:
http://haxer.skinyourscreen.com and go to the Video Tutorial link. I am in the process of making a video tut series that shows how to completely construct in photoshop and apply a custom theme for Aston Shell.