I will rewrite a sketch from the Ethernet Shield for WiFiShield. Find no help.

I am looking for 3 weeks after the tutorials and instructions I want to rewrite a sketch, which works fine with the Ethernet Shield, for the WiFiShield. I also can not find a description of the differences between the Ethernet Library and the WiFiLibrary. It stands just everywhere, that it makes little effort to adjust this. But nowhere stands exactly what and how. And why. I've tried many times to assemble two sketches. One for Ethernet and one for the Wifi. But does not work at. also the simple and stubborn replace all Ethernet blocks with Wifi blocks is not successful.
Without your help, I'm stuck here.

Big fat thanks

Start by replacing Ethernet.h with WiFi.h. This will cause a bunch of compile errors. Fix the first error and try the compile again. Repeat until there are no compile errors. Then see if your program works.