Launch iTunes on your computer and go to Store>View my account. Enter your password at the prompt. Go down to iTunes in the Cloud. Click on Manage Devices. You can remove the iPad as an associated device in there - if that is what you want to do.
Other than that - you do not need to remove your iPad from your iTunes library or deregister it in any way. You should contact the police and you might want to consider changing email passwords and any credit card information from iTunes.
This is Apple's article - Reporting a lost or stolen Apple Product.
You can take a look at that - and copied from that article ....
If you have already configured the MobileMe Find My iPhone service on your iPhone, iPad, or iPod touch, you can use it to locate your missing device, display a message on its screen, make it play a sound at full volume (even if on mute), remotely lock it, or remotely wipe (erase) it to protect your privacy. Please see the following articles for more information:
If your mobile device is lost or stolen, see this article to find your iTunes-purchased files.