There a re lost of theme Available for iPhone these days, but you want to create your own theme and don,t know how to make it. Then you have to try iPhone Theme Maker. Which allows you to create iPhone themes easily online. Its a very simple online tool.
It also lets you customize your theme by adding a wallpaper, and changing the dock background and status image. You can upload your own images from the computer.
I know http://www.iPhoneThemeGenerator.com and it is simpler