Hey, I'm a theme developer. I'll help you as much as I can in this post.
The first step you should take is SSH'ing into your device, copying over some themes and seeing how they work. Most customization is done in the Bundles folder, while some is done in the UIImages folder.
Dimensions and filenames are important too.
Download this iOS 5.1 theme, it's a great way to have reference images, dimensions and filenames(although it usually says ~iphone@2x when it's meant to be @2x~iphone, you'll figure out what I mean). Be aware however that some images aren't used anymore and some may have different names.
I also recommend downloading the full Glasklart theme, it's by far the most complete theme I've ever found, so if you're not sure where to look for an image or what a certain bundle identifier is, you can easily look it up.
You should check out some theme groups on deviantART for inspiration, there are some amazing designers out there who don't know how to make themes, and I'm sure they'd appreciate someone who knows what they're doing!
Feel free to ask me any more questions, I'll answer them to the best of my ability.