Unfortunately, the RPi cannot act as a USB device for the iPad, it can only work as a USB host.
One possible option is to use the RPi to build something like a MiFi (http://en.wikipedia.org/wiki/Mifi
). Add a Verizon modem and a WiFi adaptor to the RPi and configure the RPi as a router. Devices in your house (like your iPad) can then connect to the internet using WiFi.
The details are not suitable for the "Absolute beginners" forum, but I have built several similar systems using 3G modems on smaller ARM machines than the RPi.
You need to buy an LTE modem which is easy to use with Linux. You also need a WiFi adaptor which can work in host mode. Some research will be needed, if you are not adventurous I recommend waiting for others to experiment first.